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− Ghimbi Journal of Mineralogical Petrological Sciences, Volume 103, page 23 ─ 46, 2008 Petrogenesis and of the Neoproterozoic Bikilal gabbro

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Petrogenesis of the Neoproterozoic Bikilal-Ghimbi gabbro, Western Ethiopia Binyam W. WOLDEMICHAEL*, ** and Jun−Ichi KIMURA* * Department of Geoscience, Shimane University, Matsue 690 −8504, Japan On leave from Geological Survey of Ethiopia, P. O. Box 2302, Addis Ababa, Ethiopia

**

The Western Ethiopian Shield is an exposed Neoproterozoic metamorphic belt and forms part of the Arabian− Nubian Shield. The metamorphic belt consists of high−grade biotite gneisses, low−grade volcanogenic sediments, and mafic −ultramafic complexes. The Bikilal − Ghimbi gabbro is a mafic body surrounded by these gneissic rocks, and is located 440 km west of Addis Ababa. The gabbro is elliptical in shape and covers an area of 350 km2. It consists of olivine gabbro in its center and hornblende gabbro and hornblendite at the perimeter. The olivine gabbros are very fresh and undeformed, but hornblende−bearing suites have deformational textures. Each rock type can be divided into apatite−bearing and apatite−free subtypes. The major element geochemistry shows that despite the differences between the olivine and the hornblende gabbros, there is no systematic chemical contrast between the lithotypes except for the fluid mobile elements, suggesting an origin from a common parental magma. Only the perimeter is affected by metasomatism. An estimation of the parental magma composition using the trace element abundance in fresh clinopyroxenes and fresh olivine gabbro bulk rock suggests an intraplate−type tholeiite. Crystallization model calculations using a tholeiitic parental magma suggest that the gabbros crystallized in a manner where small amounts of interstitial melt were retained. The apatite−bearing varieties are always associated with Mg−rich mineral phases, suggesting an origin from the supercooling of replenished basalt into an evolved low temperature magma chamber. The supercooling caused saturation of the apatite in the basalt melt, along with Mg−rich crystals, and these later mixed together with the more evolved crystals that had precipitated previously. The intraplate−type tholeiitic parental magma suggests plume−type magmatism for the origin of the Bikilal− Ghimbi gabbro body. Keywords: Bikilal− Ghimbi gabbro, Geochemistry, Neoproterozoic, Tholeiite, Parental magma, Petrology, Ethiopia

INTRODUCTION Studies on mafic intrusions, e.g., those covering their parental magma composition, their movements upward through the crust, and their differentiations and modifications after being emplaced in the crust, are critical to an understanding of the development of the Earth’s crust. A common problem faced in igneous petrology is that many intrusive rocks are not quenched liquids, but are crystal segregations or cumulates (e.g., Wager and Brown, 1968; Irvine, 1982; Hunter, 1996). The chemistry of cumulates in plutonic rocks is principally controlled by the modal proportions and composition of the minerals that have been accumulated and the interstitial melt entrapped during crystallization (Bédard, 1994; Green 1994). Whole−rock trace element analyses can be used in doi:10.2465/jmps.070401 B.W. Woldemichael, [email protected] Corresponding author

conjunction with the partition coefficients and mineral modes to calculate the liquidus−temperature equilibrium distribution of the trace elements among the constituent minerals of the cumulate and plutonic rocks (Bédard, 1994; Green, 1994; Wood and Blundy, 1997). These data can also provide information on the parental magma types, and thus, the tectonic implications of gabbro bodies (e.g., Bédard, 2001; Claeson and Meurer, 2004; Kumar et al., 2007). This is particularly important in studies on gabbros in old terrains. The Western Ethiopia Shield is a Neoproterozoic metamorphic belt that forms part of the Arabian−Nubian shield (ANS, Abdelsalam and Stern, 1996). The metamorphic terrain is underlain by high−grade gneisses and by low−grade metavolcanics and metasediments with associated mafic −ultramafic belts and syn − to post−tectonic gabbroic to granitic intrusions. The Bikilal− Ghimbi gabbro is elliptical in shape, and has a concentric structure,

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with olivine gabbro in its center and hornblende gabbro and hornblendite at its periphery. The Bikilal− Ghimbi gabbro is regarded as a syn− to post−tectonic intrusion, based on its relationship with the surrounding country rocks (Abraham, 1989; Alemu and Abebe, 2000; Beshawered, 2001; Allen and Tadesse, 2003). Previous studies have regarded the gabbro as being two separate bodies, namely a syn−tectonic hornblende gabbro and a post−tectonic olivine gabbro. Several geological mapping and mineral exploration programs on different scales have been conducted in the Bikilal− Ghimbi area that have attempted to find economic deposits of iron and phosphorus. However, no petrological and geochemical investigations that could constrain the petrogenesis of the gabbroic body have yet been carried out. This work aimed to investigate the petrology and geochemistry of the Bikilal− Ghimbi gabbro to describe the mineralogical and geochemical variations within it, and to clarify the petrogenesis of the intrusion, including the nature of the parental magma and apatite ore genesis. The results show that: (1) the crystallization sequence includes fractional crystallization of olivine, plagioclase, and clinopyroxene and primary basalt replenishment to form apatite ores within the Mg−rich layers; (2) the hornblende gabbro and hornblendites are metasomatized equivalents of the olivine gabbro; and (3) the major and trace element analyses of fresh clinopyroxenes and bulk olivine gabbros suggests a tholeiitic parental magma in composition. These results shed light on the igneous and tectonic processes in the West Ethiopian Shield and in the ANS. GEOLOGICAL SETTING OF THE BIKILALGHIMBI GABBRO Regional geology The term “Pan−African” (Kennedy, 1964) originally referred to a sequence of tectonothermal events at 500 ± 100 Ma within Africa and Gondwana. However, Kröner (1984) included orogenic events in the period 950− 450 Ma, and identified the Arabian −Nubian Shield (ANS) as one of the major orogenic belts formed during Neoproterozoic time. The “Pan−African” assembly of Gondwana is thought to be the cause of the East African Orogen, which included a complex intraoceanic and continental margin magmatism and a protracted sequence of tectonothermal events (Stern, 1994). Two major terrains have been identified in the East African Orogen. The first is the juvenile (i.e., mantle derived) ANS in the north, where the ANS is dominated by low−grade volcano−sedimentary rocks associated with plutons and ophiolitic remnants (Fig. 1a). The second is a tract of older remobi-

lized crust to the south of the ANS, known as the pre− Neoproterozoic Mozambique Belt (MB, Fig. 1a). This is dominated by high−grade gneisses and migmatites. The East African Orogen stretches south from Israel and Jordan through to Tanzania and into Antarctica (Stern, 1994). The transition between the juvenile ANS and the MB is located within Ethiopia, Eritrea, Sudan, and Somalia. The Western Ethiopian Shield (WES) lies near the transition between the ANS and the MB, and also lies adjacent to the enigmatic “East Saharan Metacraton”, which consists of older crust that was extensively remobilized during Neoproterozoic time (Abdelsalam et al., 2002). There has been a limited number of studies on the geochronology and magmatic geochemistry of the WES, and those that have been carried out have shown results that are mostly in accordance with models derived for other parts of the ANS. Three generations of plutonism are recognized in the WES: (1) “prekinematic” plutons yield emplacement ages of 814−866 Ma (Ayalew et al., 1990; Kebede et al., 2001; Grenne et al., 2003); (2) “synkinematic” plutons yield emplacement ages of 700−783 Ma (Ayalew et al., 1990; Kebede et al., 2001); and (3) “late− to postkinematic” plutons yield emplacement ages of 541− 625 Ma (Ayalew et al., 1990; Kebede et al., 2001). Ayalew and Peccerillo (1998) suggested that the pre− to synkinematic plutons were subduction related, and were emplaced in intraoceanic island−arc environments. Ayalew and Peccerillo (1998) and Kebede et al. (1999) inferred that “late− to postkinematic” plutons possessed geochemical signatures of both subduction−related and intraplate components. Allen and Tadesse (2003) classified and described the N−S trending Tuludimtu Belt, where the area of study of this work lies, into five lithotectonic domains. They interpreted the belts as being a collision of the orogenic belt formed during the assembly of west Gondwana before the final closure of the Mozambique Ocean. According to this tectonic subdivision, the Bikilal− Ghimbi intrusion lies in the Didesa Domain of these five lithotectonic domains (Fig. 1b). The rocks within the Didesa Domain consist of moderate grade paragneisses and orthogneisses intruded by Neoproterozoic intrusive rocks. The paragneisses consist of interlayered biotite amphibole gneiss, garnet−biotite gneiss, quartzitic gneiss, and very coarse granitoid gneiss. The orthogneisses are represented by a banded mafic gneiss containing ultramafic bands, and could have been derived from a layered mafic intrusive body. With the exception of the quartzofeldspathic gneiss, which is relatively massive, all are strongly foliated and have abundant refolded folds indicating two or more generations of folding. These rocks have undergone several later brittle de-

Petrogenesis of the Neoproterozoic Bikilal− Ghimbi gabbro

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Figure 1. (a) A generalized map of the ANS showing the regional pre−Neoproterozoic crust, Neoproterozoic crust (juvenile and remobilized), and possible suture zones/ophiolites. A part of the MB is indicated (modified after Abdelsalam and Stern, 1996; Worku and Schandelmeier, 1996). (b) A geological map of part of the Tulu−Dimtu orogenic belt in the Ghimbi region (modified after Allen and Tadesse, 2003). Three of the five litho −tectonic domains are indicated. (c) A geological map of the Bikilal− Ghimbi gabbroic intrusion and the location of the sampling sites and the cross−section A−B (modified after Beshawered, 2001).

formation episodes, manifested by complex vein arrays filling fracture networks. The gneisses appear to be intruded by a series of weakly deformed igneous rocks, including gabbro, granodiorite, and granite, all of which display a steep N−S foliation, which is concordant with

the extension direction of the belt. Few ages are available for the WES gneisses. Johnson et al. (2004) dated 830−785 Ma for the emplacement/ crystallization of the igneous protoliths to the orthogneisses of the WES. The age and affinity of the poly-

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deformed gneisses in western Ethiopia are still ambiguous, and at present, no chronological data constrain the ages of the mafic intrusions in western Ethiopia. The Bikilal-Ghimbi gabbroic intrusion Several gabbroic intrusions occur within the WES. Based on their occurrence, these are classified into syn −, late−, and post−tectonic varieties. The Bikilal− Ghimbi gabbroic intrusion is one of the largest intrusions in the region, and is considered to be a syn− to post−tectonic type, based on its apparently intrusive relationship with the surrounding gneiss (Abraham, 1989; Alemu and Abebe, 2000; Beshawered, 2001; Allen and Tadesse, 2003). The intrusion underlies an area of about 350 km2, and is about 40 km long, with a maximum width of about 14 km in its central part and a minimum width of 2 km at its southern tip (Figs. 1b and 1c). An area of about 70 km2 is underlain by olivine gabbro, whereas the remainder mainly consists of hornblende gabbro with layers of hornblendite. The gabbro defines an elliptical area that is elongated parallel to the N−S foliation of the surrounding gneissic terrain. The contact with the gneissic terrain is thought to be an intrusive type, although no chilled margins or dikes of gabbro in the gneiss are observed. Layering is a common feature of gabbroic intrusions. Individual layers or layered sequences can vary greatly in thickness, texture, shape, and in their mineralogical and chemical composition (Naslund and McBirney, 1996); the Bikilal− Ghimbi intrusion is no exception. The olivine gabbro body has no clear layering. However, an intensive layering is only limited in the periphery, which mostly consists of hornblendites and hornblende gabbros, whereas the olivine gabbro cores are apparently massive. Layering/banding features occur in the hornblende gabbro. The olivine gabbro shows some variation, especially on pyroxene grain size level. Pyroxene crystals as large as 2 to 3 cm characterize the coarser varieties, whereas the equivalent finer varieties contain relatively fine −grained pyroxene (1−5 mm). This textural variation can be considered as textural layering, although clear layering boundaries are not noticeable. Rhythmic layering is common in the northern part of the intrusion. Macrorhythmic layering (1 m to > 100 m) is manifested by hornblendite occurring as separate units within the hornblende gabbro. Microrhythmic layering, a few centimeters thick, is seen in some hornblende gabbros. The microrhythmic mafic and felsic layers (bands) contain concentrations of hornblende and plagioclase, respectively. The hornblende gabbro also varies from leuco−, meso− to melanocratic, defining a range of light (plagioclase rich) to dark (hornblende rich) bands. The ig-

neous layering in the western part of the hornblende gabbro dips 45 to 75° toward the center of the intrusion. In the eastern part, the layering follows the general strike of the intrusion and dips steeply to the west. Hornblendite layers/bodies occur frequently in the northern and northeastern parts, are less common in the south, and are rare in the central, western, and northwestern parts of the Bikilal− Ghimbi gabbro. Hornblende gabbro is also characterized by an Fe−Ti and apatite mineralization. These mineralizations also occur within the hornblendite layers. SAMPLES AND ANALYTICAL METHODS Samples Eighty−nine unweathered fresh samples were analyzed. Of these, 72 were collected from surface outcrops, 16 were drill core samples, and one additional surface sample was drawn from the core and outcrop archives of the Geological Survey of Ethiopia. Of the samples analyzed, 32 were apatite−free olivine gabbros, 35 were hornblende gabbros, and two were hornblendites. Apatite−bearing equivalents were represented by three, six, and five samples, respectively. Three additional samples from the surrounding gneisses were also analyzed, along with three samples from the granitic intrusions. The outcrop sample numbers (Fig. 1c) were prefixed by the abbreviation BG (e.g., BG 16), except for the two samples preceded by the codes TCH and POG. The drill core samples were preceded by either of the numbers 512 or 610. A 1:50000 scale geological map of the Ghimbi area was used for the sampling (Fig. 1c), which was produced by an agro−mineral exploration project team that included the first author of this work and was later compiled by Beshawered (2001). Bulk rock analysis The bulk rock major and trace element compositions of all the samples were determined using X−ray fluorescence (XRF) spectrometry employing glass disks. The samples were crushed into small chips using a hammer, rinsed with distilled water in an ultrasonic bath, and dried for a period of 2 h at 100 °C. The dried chips were then ground in an agate mortar. The resulting powders were ignited in a muffle furnace for a period of 3 h at > 1000 °C to determine the loss on ignition. The ignited samples were then used to prepare glass beads containing a mass of 1.8 g of the powder and 3.6 g of an alkali flux, following the method of Kimura and Yamada (1996). The alkali flux used was a mixture of lithium tetraborate and lithium metaborate in a ratio of 4:1. The glass beads were analyzed for the major elements and 11 trace elements using

Petrogenesis of the Neoproterozoic Bikilal− Ghimbi gabbro

a Rigaku RIX 2000 XRF spectrometer located at Shimane University, Japan. The trace and ultratrace element analyses of 16 samples of the olivine and hornblende gabbros and their apatite−bearing equivalents were carried out using solution− based Inductively Coupled Plasma−Mass Spectrometry (ICP−MS) employing the method described by Kimura et al. (1995). The ICP−MS used was a Thermo Elemental VG PQ3 located at Shimane University, Japan.

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Table 1. Modal analyses (vol%) of the different rock units

Mineral analysis Petrological descriptions and modal analysis were carried out on 23 samples, including four hornblendites, eight hornblende gabbros, and 11 olivine gabbros. The major element mineral chemistry of representative plagioclase, olivine, pyroxene, hornblende, and oxide minerals was determined using a JEOL JXA−8800M electron microprobe located at Shimane University, Japan, using an accelerating voltage of 15 keV and a beam current of 2 × 10−8A. Data correction followed the ZAF method (e.g., Reed, 1975). The diameters of the focused beam for the analyses were 5 μm for the olivine, plagioclase, and the oxide minerals, and 20 μm for the pyroxene. The trace element abundance of several minerals was analyzed using an in−house 193−nm excimer laser ablation microprobe combined with the VG PQ3 ICP−MS (Kimura et al., 2000). The mass peak occurring at 44Ca was used as an internal standard. The analysis included 45 spots on the clinopyroxenes. Three adjacent points were analyzed, and the resulting data were averaged. The major element compositions of the analyzed grains were determined using the EPMA before analysis using the LA−ICP−MS were carried out. The instrument setting used for the pyroxenes a pulsed laser with 200 mJ of laser output power. The laser craters formed were ~ 60 μm in diameter with depths of ~ 30 μm after an ablation time of 60 s. PETROGRAPHY Hornblendite The hornblendite is fine − to medium−grained with phaneritic and massive texture. The hornblende grains are subhedral to anhedral, and show slight schistosity. Uralitization is prominent, with rare traces of unaltered pyroxene cores. The hornblendite is subdivided into apatite−bearing (modal apatite > 3%) and apatite−free groups (modal apatite ≤ 3%). The “apatite−free groups” have no discernible apatite. The modal composition of the hornblendite ranges from 60 to 90% hornblende, 5 to 30% ilmenite−magne-

Ol Gb, olivine gabbro (apatite free); Ol Gb Ap, olivine gabbro apatite bearing); Hb Gb, hornblende gabbro (apatite free); Hb Gb Ap, hornblende gabbro (apatite bearing); HBT, hornblendite (apatite free); HBT AP, hornblendite (apatite bearing); Ol, olivine; Cpx, clinopyroxene; Hb, hornblende; Pl, plagioclase; Ox, oxide minerals; Ap, apatite.

tite, 0 to 9% plagioclase, and 0 to 20% apatite (Table 1). The hornblendite units are hosted within the hornblende gabbro. Hornblende gabbro The hornblende gabbro is the outer unit and formed a large portion of the Bikilal− Ghimbi intrusion. This unit is characterized by melanocratic to leucocratic varieties, depending on the plagioclase and hornblende content, and is generally medium to coarse grained. This unit also hosts hornblendite. The plagioclase and hornblende exhibit a relict to granoblastic texture, with accessory ilmenite, apatite, and chlorite. The hornblende gabbro is composed of 40 to 70% hornblende, 0.5 to 15% ilmenite−magnetite, 20 to 55% plagioclase, and 0 to 20% apatite (Table 1). This unit is also subdivided into apatite−bearing and apatite− free hornblende gabbro. The apatite−bearing gabbro commonly occurs within the hornblende gabbro mass, and locally, is regarded as disseminated apatite mineralization. The plagioclase shows fine albite twining and a kink structure had developed within the crystals, indicating that the rocks had undergone deformation. In a few places, the plagioclase shows a granular texture and is

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coarse−grained holocrystalline, with a hypidiomorphic granular gabbroic texture, and the modal compositions range from 5 to 25% olivine, 10 to 35% clinopyroxene, 30 to 55% plagioclase, 0 to 16% apatite, and 1 to 15% ilmenite−magnetite (Table 1). Most of the crystals are subhedral to anhedral, but most of the apatite is euhedral. The olivine is interstitial with ophitic to subophitic plagioclase in the clinopyroxene oikocrysts (Fig. 2a). All the crystals are weakly zoned. This unit is further classified into apatite − bearing and apatite−free olivine gabbro. The olivine gabbro is essentially massive, and may contain coarser pyroxene crystals (2 to 3 cm in size). The fresh nature of the olivine gabbro allows us to investigate the origin of the Bikilal− Ghimbi gabbro further using geochemistry. Therefore, the geochemical interpretations described in later sections in this paper are based on data from the unaltered olivine gabbro. GEOCHEMISTRY Major element bulk rock chemistry

Figure 2. Representative photographs of thin sections of the Bikilal− Ghimbi gabbro: (a) fresh olivine gabbro and (b) hornblende gabbro with apatite and ilmenite−magnetite. Remnants of pyroxene are visible, indicating the hornblende was originally pyroxene. Pl, plagioclase; Cpx, clinopyroxene; Ol, olivine; Hbl, hornblende; Ilm, ilmenite; Mgt, magnetite; Ap, apatite; Alt, altered.

broken down to form fine grains, from which the original twining is lost. The hornblende appears to be an altered/ metasomatized product of the pyroxene, as suggested by some hornblendes that contain traces of pyroxene (Fig. 2b). These observations, combined with the gradational contact with the olivine gabbros seen in some places, suggest a hydrothermal alteration/metasomatism at late stage magmatism may have played a role in the formation of the hornblende gabbro. Olivine gabbro A large mass of olivine gabbro occupies the central portion of the Bikilal− Ghimbi intrusion. This massive olivine gabbro is surprisingly fresh and is unaltered, except for minor veins and weathering. Deformation textures are rare, and all the crystals are fresh and retained their original igneous shape. The olivine gabbro is medium− to

The major element geochemistry of the Bikilal− Ghimbi gabbro is based on analyses of 36 olivine gabbros (including a subset of three apatite−bearing samples), 45 hornblende gabbros (seven apatite−bearing), and seven hornblendites (five apatite −bearing). Hereafter, we will apply the above descriptors to the three rock units. Representative bulk rock major and trace XRF data are shown in Table 2, and all the analyzed samples are tabulated in Appendix 1. In the Na2O + K2O versus SiO2 classification diagram of Cox et al. (1979), as modified for plutonic rocks by Wilson (1989), the Bikilal− Ghimbi gabbro plots almost entirely in the subalkalic magma series field, and has a gabbroic composition (Fig. 3). The silica content of the apatite−free gabbros and hornblendites shows a wide range from 45 to 55 wt%. The apatite−bearing subsets all plot out of range on this diagram due to their low silica content (25− 45 wt%, not shown in Fig. 3). This is due to the effect of the apatite concentration, which results in an abundance of P2O5 of up to 8 wt%. Although rock types range from fresh olivine gabbro to uralitized hornblende gabbro to hornblendite, overall, the Bikilal− Ghimbi gabbro samples fall in a narrow compositional range. Significant variations in FeO, TiO2, and P2O5 are observed in the Bikilal− Ghimbi gabbro. The AFM triangular plot shows an Fe enrichment trend for the apatite− bearing subsets. The boundary proposed by Kuno (1968) reveals both tholeiitic and calc−alkaline trends (Fig. 4), whereas only a tholeiitic trend is observed according to the boundary proposed by Irvine and Baragar (1971).

Ol Gb, olivine gabbro (apatite free); Ol Gb Ap, olivine gabbro (apatite bearing); Hb Gb, hornblende gabbro (apatite free); Hb Gb Ap, hornblende gabbro (apatite bearing); HBT, hornblendite (apatite free); HBT AP, hornblendite (apatite bearing).

Table 2. Representative whole−rock analyses from the Bikilal− Ghimbi gabbroic intrusion

Petrogenesis of the Neoproterozoic Bikilal− Ghimbi gabbro 29

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Figure 3. A total alkali versus silica diagram (Cox et al., 1979, adopted for plutons by Wilson, 1989). Only apatite−free samples are plotted, as apatite−bearing samples are poor in both silica and alkali, and are plotted out of scale.

The MgO content of the Bikilal− Ghimbi gabbro ranges from 2 wt% to 13 wt%. The MgO abundance overlaps between rock suites, irrespective of the apatite content (4−12 wt% for the olivine gabbro, 2−12 wt% for the hornblende gabbro, and 8−13 wt% for the hornblendites) (see Fig. 5). Although there is little difference in the range of MgO in the olivine and hornblende gabbros, most of the olivine gabbros are characterized as having a higher MgO content (8−13 wt%), whereas the majority of the hornblende gabbros are low in MgO (2−8 wt%) for both the apatite−bearing and −free subsets. A plot of MgO versus P2O5 highly discriminates between the apatite−bearing and apatite−free groups (Fig. 5). The apatite−bearing subsets are enriched in P2O5 (2−8 wt%), whereas the apatite−free subsets contain very little P2O5 (0.1− 0.3 wt%). The magnesium numbers also range from highly evolved, with values as low as Mg# = 15, to slightly evolved (Mg# = 55). Most olivine gabbros have Mg# = 50−55. The modal enrichment of the Fe−Ti oxides in the gabbro led to a lower Mg#. MgO correlates positively with FeO, TiO2, MnO, and P2O5 in the apatite−bearing subsets, and correlates negatively with Al2O3, SiO2, K2O, and Na2O. Both the apatite− free and the apatite−bearing olivine gabbros show strong negative correlations of Na2O and K2O with MgO. A slightly negative correlation is observed for Al2O3 and SiO2 in the apatite−free lithotypes. FeO, TiO2, and MnO in the apatite−free olivine gabbros increase slightly with increasing MgO up to ~ 7 wt%, and then they decreased with increasing MgO. The chemical composition of the apatite−free hornblende gabbros and hornblendites over-

Figure 4. An AFM classification diagram for the Bikilal − Ghimbi gabbro showing both calc−alkaline and tholeiitic trends according to the boundary between the two fields, as proposed by Kuno (1968), showing the tholeiitic trend proposed by Irvine and Baragar (1971).

lapped extensively with that of the apatite−free olivine gabbros, but the data are more scattered. This is particularly true for K2O. The K2O abundance in the hornblende gabbros is up to about five times greater than that in the other lithotypes (Fig. 5), suggesting a metasomatic addition of potassium. Most of the hornblende gabbros with the highest K2O contents lie either close to the contact with the gneiss (e.g., Sample BG 20) or along the margins of the regional lineaments/faults (e.g., Samples BG 76, BG 77, and BG 82; Fig. 1c). The apatite−bearing subsets have greater modal apatite and iron oxide contents (18−30 modal percent in the olivine and hornblende gabbros, and 30− 47 modal percent in the hornblendites; see Table 1). The difference in modal composition is also reflected in the low SiO2, Al2O3 content, and the higher MnO abundance in the apatite−bearing samples. Major element mineral chemistry The mineral chemistry of the olivines, pyroxenes, plagioclases, hornblendes, and the oxide minerals of the Bikilal− Ghimbi gabbro was examined based on 542 spot analyses. Most of the analyses were carried out on the olivine gabbro, because it was the freshest lithotype and was virtually unaltered. Representative results are shown in Tables 3−7, and all the data from the analyzed spots are

Petrogenesis of the Neoproterozoic Bikilal− Ghimbi gabbro

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Figure 5. Major element−MgO variation diagrams for the Bikilal− Ghimbi gabbro. The end member was estimated using multiple least square regression calculations.

tabulated in Appendix 2. Olivine. Olivine constitutes 5−25% of the olivine gabbros. The olivine has Mg numbers [Mg# = 100 Mg/(Mg + Fe)] ranging from 45 to 75 (Fo45−75). Representative olivine analyses are listed in Table 3. The olivine is normally homogeneous from core to rim with no obvious zoning. The Fo content varies little within individual samples (e.g., Sample BG 54; Fo 70−74), whereas a higher variation was observed between different samples (e.g., Samples BG11 and TCH 23, Fo60−76). The olivine composition showed a trimodal distribution, with a median Fo content at Fo49.2, Fo61.1 and Fo 72.4 corresponding to a host bulk rock MgO content of 6 wt%, 6−9 wt%, and 10−11 wt%, respectively. The olivine exhibits a variable Fo content, especially with evolved compositions in the apatite−bearing samples (Fo40−60). This shows that the span of olivine crystalliza-

tion was prolonged. Given that the Ni content of the olivines is very low (< 0.1 wt%) and the Fo content is low to moderate (Fo40−70), it appears that the Bikilal− Ghimbi gabbro crystallized from an evolved magma. Pyroxene. Clinopyroxene is present throughout the olivine gabbros and constitutes 25 to 35% of the modal composition. Representative clinopyroxene analyses are shown in Table 4. Compositionally, the clinopyroxenes are mainly augite (34−52 mol% En, 10−27 mol% Fs, and 32− 45 mol% Wo), with little diopside present (42− 48 mol% En, 10−19 mol% Fs, and 45− 47 mol% Wo). The pyroxenes exhibit a limited Fe enrichment relative to olivine, with Mg−numbers in the range 69−80 (Mg# = 45−75 in olivine). The grains are homogeneous from core to rim, with no identifiable zoning. The value of the Mg# varies considerably within individual samples (e.g.,

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B.W. Woldemichael and J.−I. Kimura Table 3. Representative olivine analyses

Ol Gb, olivine gabbro (apatite free); Ol Gb Ap, olivine gabbro (apatite bearing).

Sample BG 11 with Mg# = 77−73), but not within grains. However, the variation in Mg# was even higher between different samples (e.g., Samples 6104 and TCH 23, with Mg# = 69−80). The value of the Mg# of the clinopyroxenes and the range in silicon content (1.82−1.95 pfu) is typical of subalkaline gabbro (e.g., Tepper, 1996). The concentration of the minor elements, such as NiO, is very low (< 0.07 wt%), whereas the Cr2O3 content showed more variation. Some clinopyroxenes from both apatite−free and apatite−bearing suites have very low Cr2O3 content (< 0.06 wt%). Other clinopyroxenes mostly from the apatite−free suites contain 0.1− 0.5 wt% of Cr2O3. These Cr−rich clinopyroxenes may have crystallized at an early stage before the parental magma was depleted in Cr2O3. Plagioclase. Plagioclase occurs in both the olivine gabbro and the hornblende gabbro. Representative plagioclase

Table 4. Representative clinopyroxene analyses

Ol Gb, olivine gabbro (apatite free); Ol Gb Ap, olivine gabbro (apatite bearing).

analyses are given in Table 5. The plagioclase from the olivine gabbro ranges in composition from An44−72. The plagioclase grains are generally homogeneous from core to rim, with no evident zoning. Thus, the compositional variation within each sample is relatively small (e.g., Samples BG 54; An60−62) compared to that between samples (e.g., Samples BG11 and TCH 23, An45−70). The plagioclase composition shows a trimodal distribution, with medians at An46.7, An55, and An63. Most of the plagioclase in the apatite−bearing samples (e.g., Samples 6104 and 5125) is less than in An46.7 and is more sodic than that from the apatite−free varieties. Although the number of plagioclase analyses carried out on the hornblende gabbro was insufficient for statistical examination, they do show a slightly wider range of An content than those from the olivine gabbro (e.g.,

Petrogenesis of the Neoproterozoic Bikilal− Ghimbi gabbro Table 5. Representative plagioclase analyses

33

Table 6. Representative hornblende analyses

Hb Gb, hornblende gabbro (apatite free).

Ol Gb, olivine gabbro (apatite free); Ol Gb Ap, olivine gabbro (apatite bearing); Hb Gb, hornblende gabbro (apatite free).

Table 7. Representative oxide minerals analyses

Samples BG57 and BG35, An42−75). Like the plagioclase of the olivine gabbro, the plagioclase in the hornblende gabbro do not show any zoning. Amphibole. The uralitized hornblendes were difficult to analyze using the microprobe because of their secondary origin and their derivation from primary pyroxene. Despite the difficulty in analyzing the uralitized hornblendes, several amphibole analyses with acceptable stoichiometry were acquired. Representative results are listed in Table 6. The amphiboles in the Bikilal− Ghimbi hornblende gabbro are calcic amphiboles, ranging from tschermakite and magnesiohornblende to actinolite hornblende, according the classification of Leake et al. (1997). Most are magnesiohornblende. The amphiboles were characterized by high Al2O3 contents (6−12 wt%). The XMg values [XMg = Mg/(Mg

Ol Gb, olivine gabbro (apatite free); Ol Gb Ap, olivine gabbro (apatite bearing); Hb Gb, hornblende gabbro (apatite free); Mag, magnetite; Ilm, ilmenite.

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Figure 6. Trace element−MgO variation diagrams for the Bikilal− Ghimbi gabbro.

+ Fe2+)] are > 0.5, and range from 0.6 to 0.8. Their Cr2O3 content ranges from < 0.01 wt% to 0.8 wt%, and a high Cr2O3 abundance corresponds with a high XMg value. This shows that Mg and Cr abundances in the olivine gabbro protoliths were variable, and this variability was inherited by the amphiboles. The optical evidence suggests that the origin of the hornblende is from the pyroxene (Fig. 2b), but the geochemical process involved could not be constrained. Most of the alteration seen is uralitization, and uralitization to form hornblende can be associated with late stage hydrothermal recrystallization (Deer et al., 1992). Oxide minerals. The oxide minerals in the Bikilal− Ghimbi gabbro are ilmenite (92−97 mol% Ilm) and minor magnetite (86−99 mol% Mgt) (Table 7). The Cr2O3 abundance in the magnetite is generally very low, but several analyses from the most magnesian olivine gabbros show

higher levels (2 to 3 wt% Cr2O3). Trace element bulk rock chemistry All the samples analyzed for major element analysis were also analyzed for 11 trace elements using XRF (Table 2). The Ba, Sr, and Ga contents correlate negatively with MgO, and the chemical variation between the apatite−free and apatite−bearing samples is similar, whereas in the trends for other trace elements, the apatite−free and apatite−bearing gabbros are more distinct (Fig. 6). The apatite−free olivine gabbro shows clearer correlations than the hornblende gabbro did, whose data tend to be scattered. The incompatible trace elements of Ce and Y correlate positively with MgO in the apatite−bearing subset, and their abundance is elevated due to the concentration of rare earth elements (REE) and yttrium in the apatite (Henderson, 1984). Both Ni and Cr show positive correla-

Ol Gb, olivine gabbro (apatite free); Ol Gb Ap, olivine gabbro (apatite bearing); Hb Gb, hornblende gabbro (apatite free); Hb Gb Ap, hornblende gabbro (apatite bearing).

Table 8. Trace and REE element analyses of the Bikilal− Ghimbi intrusion

Petrogenesis of the Neoproterozoic Bikilal− Ghimbi gabbro 35

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B.W. Woldemichael and J.−I. Kimura

Figure 7. Primitive mantle normalized (Sun and McDonough, 1989) trace element spider diagrams of the Bikilal− Ghimbi gabbro: (a) olivine gabbro (apatite free); (b) hornblende gabbro (apatite free); (c) olivine gabbro (apatite bearing); and (d) hornblende gabbro (apatite bearing). The shaded regions are olivine gabbro and hornblende gabbro (apatite free) from Figures 7a and 7c, respectively.

tions with MgO in the apatite−free olivine gabbro and hornblende gabbro, and are almost constant over the entire MgO range in the apatite−bearing samples. In contrast, the V content is high, and it correlates with the MgO content in the apatite−bearing samples, whereas the V content is low and varies similar to the TiO2 and FeO content (see Fig. 5) in the apatite−free olivine gabbro. The decrease in Ni content with decreasing MgO content suggests an olivine fractionation. The same pattern for Cr suggests a clinopyroxene fractionation. The low Cr and Ni contents of the apatite−bearing samples cannot be accounted for by apatite and Fe−Ti oxide concentration alone, suggesting a depletion of these elements in the magma. A similar behavior between V, FeO, and TiO2 is suggestive of a fractionation or accumulation of Fe−Ti oxides in the apatite−bearing samples. The zirconium abundance is very low, with only a few samples’ concentrations exceeding 50 ppm. This could be due to its incom-

patibility in olivine, plagioclase, and apatite. The scattered and high abundance of Ba in both the apatite−free and the apatite−bearing hornblende gabbros implies that a substitution of K by Ba had occurred in the hornblende during metasomatism. Ultratrace element and REE analyses were carried out on 16 samples, including 11 olivine gabbros (two apatite−bearing) and five hornblende gabbros (two apatite − bearing; see Table 8), and the data were plotted on a primitive mantle−normalized trace element spidergram (Fig. 7). The spidergram patterns for the apatite−free olivine and hornblende gabbros are similar, except for elevated incompatible element abundances, particularly Cs, Rb, Pb, and Li, in the hornblende gabbro (Figs. 7a and 7b). The similarity in patterns, except for these slight enrichments, further supports the petrological findings that the hornblende gabbro suites are hydrothermally altered products of the olivine gabbro. The patterns show positive

Petrogenesis of the Neoproterozoic Bikilal− Ghimbi gabbro

37

Table 9. Representative trace and REE analyses of clinopyroxenes from the olivine gabbro of the Bikilal− Ghimbi intrusion, including analysis of some apatite and oxide minerals

Alt., altered.

spikes at Cs, Ba, K, Pb, Sr, and Eu, and negative spikes at Li, Zr, Hf, Nb, Ta, U, and Th. The samples are slightly enriched in the light REE (LREE), but the concentration level is as low as that in the primitive mantle. The apatite− bearing samples are enriched in almost all the elements relative to the apatite−free samples. Higher concentrations of Cs, Rb, and Li are also observed in the apatite−bearing hornblende gabbro. The positive spikes in Ba, Pb, Sr, and Eu may be due to a plagioclase accumulation. Low concentrations of Zr, Hf, U, Th, Ta, and Nb could reflect depletion in the source magma, but are more likely the result of a significant plagioclase accumulation due to the very low partition coefficients of these elements (Rollinson, 1993; Bindeman et al., 1998; Kimura et al., 2002). The relatively elevated patterns in the apatite−bearing samples (Figs. 7c and 7d) may be a product of apatite accumulation, as indicated by

the XRF major and trace element data. The unusual patterns shown by the Bikilal− Ghimbi gabbros do not show the melt composition, suggesting that the trace element abundance is controlled by the crystallization of specific minerals. Trace element mineral chemistry Forty−seven LA−ICP−MS spot analyses of the clinopyroxenes from the MgO−rich (9−11 wt%) olivine gabbro were carried out. (See Table 9 for representative data and Appendix 3 for all the analyzed spot data). The pyroxenes were analyzed from the core to the rim to determine if chemical zoning was present. The results suggest that the trace element composition of the clinopyroxenes is homogeneous within single grains, as observed for the major elements. With the exception of one grain, the average

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Figure 8. A primitive mantle normalized (Sun and McDonough, 1989) trace element plot of the pyroxenes from Bikilal− Ghimbi olivine gabbro. The average values of the analyzed spots were used. The numbers in brackets represent bulk MgO in wt% and the asterisk denotes an apatite−bearing subset. Altered pyroxene is denoted by the dashed line.

composition of the clinopyroxenes from four samples shows a limited variation, as observed by their near parallel patterns (Fig. 8). The REE abundance is similar, but a larger variation is observed for the high field strength elements (HFSEs; Th, U, Nb, and Ta) and the large ion lithophile elements (LILEs; Pb, Ba, and Rb). Overall, the clinopyroxene spidergram patterns show a strong depletion in LILEs and convex−upward shapes. This is similar to the behavior of clinopyroxene partition coefficients (e.g., Green, 1994). One exceptional clinopyroxene grain in Sample TCH 23 is enriched in the incompatible trace elements Rb, Ba, Th, U, and Pb (Fig. 8). Although not evident optically, the enrichment in this grain is likely to be due to trace alteration (uralitization), given that the elements above are all fluid mobile. DISCUSSION Layered structures and their origin A variety of layer−forming mechanisms can produce layering in igneous intrusions. The layering seen in the northern part of the Bikilal− Ghimbi intrusion can be explained by the combined mechanisms of convection, as seen by modal layering, and nucleation, manifested by the texture. Rhythmically banded layers with differing modal composition can result from convective circulation. In shallow magma chambers, heat is lost mainly through the roof, while crystals accumulate on the floor. This creates

buoyancy fluxes that can promote convection in the magma (Naslund and McBirney, 1996). Modally banded rhythmic layers in the northern perimeter of the Bikilal− Ghimbi hornblende gabbro may be related to such magmatic convection. Although the hornblende gabbro was slightly deformed and metasomatized, contrasts in the modal abundances of the unaltered plagioclase between leucocratic to melanocratic bands can be attributed to crystallization from convecting magma, as seen in the Skaergaard intrusion (Naslund and McBirney, 1996). Apatite−bearing bands/layers are common in the hornblende gabbros, which also exhibit modal banding. The apatite zones are more or less structurally concordant with the layering, and hence, these would also originate from a direct precipitation from the parental magma. Bikilal− Ghimbi olivine gabbro can be characterized by textural layering, as represented by the coarser and finer varieties of the pyroxene crystals. Textural layering is based on differences in textural elements (e.g., mode and grain size/shape distribution) and individual layers showing considerable internal variation. These textural variations can indicate nucleation (Hunter, 1996). The general correlation between the bulk rock MgO abundance and the Mg# of the mafic minerals and An content of the plagioclase of the gabbro suggests that overall, the gabbro was generated from the melt at different fractionation levels in the magma chamber. A differentiation in the olivine gabbro is reflected by the chemical variability found in the olivine (Fo45−75), plagioclase (An44−72), and in the clinopyroxene (Mg# = 69−80), which suggests cryptic layering. However, based on our data, the olivine gabbro body showed no systematic cryptic layering, such as a concentric structure or layered patterns. However, nucleation may have played a major role in the formation of the olivine gabbro, as implied by the texture. Crystallization sequence Olivine gabbro. As discussed above, the hornblende gabbro and hornblendite represent altered equivalents of the olivine gabbro rather than being a separate intrusion from different parental magmas. The apparent similarity of the igneous texture of the plagioclase and its close chemistry with that of the olivine gabbro indicates that substantial igneous differentiation occurred before metasomatism. This is consistent with the interpretation from the major element bulk rock chemistry that the hornblende gabbros were produced by a metasomatism of the olivine gabbro. The olivine gabbros are surprisingly well preserved and show clear igneous textures, and hence, are the best samples to examine the crystallization sequence of the Bikilal− Ghimbi body.

Petrogenesis of the Neoproterozoic Bikilal− Ghimbi gabbro

39

Figure 9. K2O content, mode abundance, and mineral chemistry of the olivine gabbro versus the bulk rock MgO content (in wt%). Pl, plagioclase; Cpx, clinopyroxene; Ol, olivine; Ilm, ilmenite; Mgt, magnetite.

Figure 10. Plagioclase anorthite (An) content versus coexisting olivine forsterite (Fo) content for establishing the tectonic setting according to the classification diagram of Beard (1986). Types I, II, and III represent different varieties of arc cumulate gabbros.

Taking into account the holocrystalline hypidiomorphic gabbroic texture observed under the microscope, the order of crystallization of the major phases is olivine, followed by plagioclase and clinopyroxene. The olivine exhibits an intercumulus texture that may reflect a liquid − solid reaction, followed by the crystallization of the clinopyroxene. The plagioclase also crystallized before the clinopyroxene, as it occurs as oikocrysts in the clinopyroxene. However, the crystallization of these three major phases should have been contemporaneous, based on the correspondence between various mineral compositions and the bulk rock chemistry. The crystallization of the major phases was followed by crystallization of Fe−Ti oxides, as these mostly occur in volume only in low MgO content samples. The olivine gabbros show linear geochemical trends (as shown by the MgO versus SiO2, Al2O3, CaO, Na2O, and K2O contents), and the inflected trends for TiO2 and FeO contents (Fig. 5), suggestive of a crystallization sequence formed along with a fractionation of the source magma. This has been tested by examination of the modal and mineral composition of the representative olivine gabbros in the MgO content range of 6−12 wt% (Fig. 9). The selected samples show a very strong correlation between the MgO and K2O contents (Fig. 9a). However, the maximum K2O content is less than 0.2 wt% K2O, an amount that could be hosted almost entirely in the plagioclase. The modal olivine and plagioclase contents do not vary significantly, whereas the clinopyroxene tends to decrease, and the Fe−Ti oxide content clearly increases with decreasing bulk MgO content (Fig. 9b). The Fo content of

the olivine and the An content of the plagioclase decreases almost linearly as the bulk MgO content decreases (Figs. 9c and 9d). These features indicate that the bulk rock composition is controlled by the mineral chemistry and modal abundances. The restricted chemical variations of each mineral, together with their euhedral to subhedral habits, strongly suggest that the olivine gabbro does not represent the melt composition. Other elements provided additional tests of the preferred crystallization sequence. The FeO, MnO, TiO2, and V abundances initially increase with decreasing MgO content. Below an MgO content of ~6 wt%, the abundance decreases as the MgO content decreases (Figs. 5 and 6). The Fe−Ti oxide minerals hosting these elements and phases are modally the most important in the MgO− poor samples. Thus, the inflected trends suggest a depletion of these elements from the parental magma due to a separation of the Fe−Ti oxides. The abundance of Ni and Cr in the olivine gabbros decline steeply with decreasing MgO content (Fig. 10). This suggests separation of Fo− rich olivine (for Ni) and of Mg−rich clinopyroxene (for Cr) from the parental magma. Such a sequence is typical in melt compositions in fractional crystallization. Hence, the chemical evolution of the parental magma and accumulations of the precipitated minerals are the prime controls of the chemical variation in the Bikilal− Ghimbi olivine gabbro samples. Apatite-bearing rocks. The apatite−bearing hornblendite, hornblende gabbro and olivine gabbro subsets have various MgO contents. As shown by the MgO−SiO2, MgO−

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Al2O3, MgO−FeO, MgO−TiO2, and MgO−P2O5 plots, the apatite−bearing suites plot along a line that converges with the apatite−free suite trend at low MgO contents (Fig. 5). This suggests the existence of an end member phase with low Si and Al contents and high Ti, Fe, Mn, P, and Mg contents (see the high Mg end member compositions in Fig. 5). Compositional control by apatite plus Fe−Ti oxides alone cannot entirely explain this variation. However, a high Mg end member composition can be achieved by mixing olivine, apatite, and Fe−Ti oxides. Thus, mixing between the evolved low Mg olivine gabbro and the high MgO end member can generate a linear trend in the apatite−bearing rocks. The extreme high−Mg end member would have a modal composition of olivine : clinopyroxene : plagioclase : magnetite : ilmenite : apatite = 29 :19 :7: 17:14 :14, assuming that the most primitive minerals observed in the olivine gabbro were the constituents. This estimate was derived by solving multiple least square regression calculations with the high Mg end member and mineral compositions. The observed percentages of apatite (17%), Fe−Ti oxide (29%), and plagioclase (3%) in the most MgO−rich apatite−bearing hornblendites (Table 1) are almost identical to those in the estimated mineral mode. Given that the uralitized hornblende is an altered product of olivine, clinopyroxene, and plagioclase, the MgO−rich hornblendite represents the metasomatized end member of the apatite−bearing olivine gabbro. A comprehensive mineral chemistry is not available for the apatite−bearing subsets. If the proposed mixing model is correct, then the mineral chemistry of the apatite−bearing subsets should be variable. As shown by the limited data, the apatite−bearing olivine gabbros (Samples 6104 and 5125), olivine (Fo44−60), and plagioclase (An44−48) are highly evolved, whereas the clinopyroxene (Mg# = 70−77) is more magnesian than the olivine. This suggests that the apatite−bearing suite rocks are a mixture between the MgO−rich and MgO−poor end members, consistent with the above mixing model. This mixing will be discussed further in a later section, along with the apatite saturation conditions. Parental magma and tectonic significance Mineralogical constraints. The composition of the coexisting plagioclase and olivine in the arc gabbros are unique to the arc setting, despite the significant variations in mineralogy and mineral chemistry present in the arc cumulate suites. Calcic (An85−100) plagioclase and moderately Fe−rich (Fo60−80) olivine commonly occur together in the arc cumulate gabbros, but do not occur together in cumulate gabbros from mid− oceanic ridges, oceanic islands, or from tholeiitic layered intrusions (Fig. 10, Beard,

1986). The An versus Fo plot implies that the Bikilal− Ghimbi olivine gabbro is unlikely to be from an arc or mid− oceanic ridge setting; rather, the gabbros may have formed in oceanic or continental intraplate settings, shown by oceanic island and tholeiitic layered intrusion fields in Figure 10. Trace element constraints. The spidergrams of the Bikilal− Ghimbi olivine gabbro (Fig. 7) do not show typical melt patterns. The spidergrams show very strong negative anomalies for the HFSEs, Th and U, with striking positive peaks for Ba, Sr, and Pb. These features may largely be due to plagioclase, which dominates the mode (Rollinson, 1993; Bindeman et al., 1998; Kimura et al., 2002). Therefore, coexisting phases are the dominant factor determining the trace and REE element chemistry of the gabbroic rocks, rather than the trapped interstitial melt. The parental magma of the crystallized end member could be further constrained by estimation of the parental melt composition. As discussed earlier, the hornblende gabbro is an altered product of the olivine gabbro, and shows a slight enrichment in fluid −mobile trace elements. Therefore, we used the fresh olivine gabbro to estimate the parental melt composition. The trace element distribution coefficients are essential for determining a magma composition from the mineral composition (e.g., Bédard, 1994; Green, 1994). The distribution coefficient (D) of a given element i is used to calculate the concentration of that element in the equilibrium liquid. The bulk value of D for element i in mineral j is calculated using the following equation: , where Φ is the modal proportion of a mineral phase. The liquid composition of element i, Liq i, can then be calculated by dividing the whole−rock concentration WRi by the bulk value of D: . If the trapped melt fraction is neglected, then in the case of cumulates, the calculations overestimate the abundance of incompatible trace elements in the minerals and coexisting liquids (Bédard, 1994). In nature, the trapped melt fraction crystallizes in the interstices between the accumulated crystals. The trapped melt crystallizes either as adcumulate overgrowths, which are difficult to recognize optically, or as discrete interstitial or reaction phases (Bédard, 1994).

Petrogenesis of the Neoproterozoic Bikilal− Ghimbi gabbro Table 10. Partition coefficients between minerals and a melt, and the modal compositions used in the model calculations

Pl, plagioclase; Cpx, clinopyroxene; Opx, orthopyroxene; Ol, olivine; Ox, oxides; Sp, spinel.

41

Based on the textural and major element data, the Bikilal− Ghimbi olivine gabbro does not contain a significant volume of trapped melt, and consequently, the trace element calculations discussed above can be applied. Published values of D for olivine, clinopyroxene, and plagioclase (Rollinson, 1993; Green, 1994; Bindeman et al., 1998; Kimura et al., 2002; see Table 10 for the values of D used in the calculations), and the modal composition of the olivine gabbros (Table 1) were used in our calculations. Magnetite and apatite were ignored, because they are rare in the MgO−rich gabbros. The bulk rock inversion is evaluated first by comparing the recalculated composition of the clinopyroxene. At pressures less than 2.5 GPa, the clinopyroxene is the principal host for the REE and, therefore, can be used to calculate the melt REE composition from the clinopyroxene composition in cumulate and plutonic rocks, where the melt is not preserved (e.g., Wood and Blundy, 1997). The spidergram patterns calculated for the clinopyroxenes (from Samples TCH 23 and BG 28) are similar to those of intraplate−type tholeiites, as represented by the Hawaiian Mauna Loa and Kilauea (Hilina) primitive tholeiitic to transitional basalts (Kimura et al., 2006), and clearly differ from N− and E−MORBs or oceanic island alkali basalts (Fig. 11a). The results show a good agreement for the REE and Rb, but the abundances of Th, U, Ta, and Nb are slightly elevated (Fig. 11a). As the D values for the Th to Nb region are very low in plagioclase compared to other minerals, these elements could be selectively concentrated in the interstitial melts. This would eventually increase the concentrations of these elements

Figure 11. A primitive mantle (Sun and McDonough, 1989) normalized plot of: (a) the calculated clinopyroxene (Cpx) composition of the liquids; the results are averaged from unaltered clinopyroxene analysis from olivine gabbro (TCH 23 and BG 28), and (b) the calculated melt composition that produces the olivine gabbros (TCH 23 and BG 28). The N−MORB, E−MORB (Sun and McDonough, 1989), and OIT fields, including the Hilina (tholeiitic and transitional) and Moana Loa (tholeiitic) after Kimura et al. (2006), are plotted for comparison.

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Figure 12. A comparison of the recalculated solid composition of the OIT simulated using the COMAGMAT software package and the observed composition of the Bikilal− Ghimbi gabbro. The low−pressure sequence of fractional (Fc) or equilibrium (Ec) crystallization fits well (Figs. 12a−12e). Figure 12f shows a solidification temperature of 1100 −1160 °C, as indicted by the box. The different symbols denote different pressures (1, 5, and 10 kbar), which can be reproduced from the initial tholeiite magma.

in the clinopyroxene. However, this process is not well constrained because of a lack of information on the interstitial melts in the gabbro. The calculated melt compositions from the bulk rock composition and the bulk values of D were normalized against those of a primitive mantle (Fig. 11b). The estimated incompatible element patterns of the calculated melt composition are also similar to those of intraplate−type tholeiite to transitional basalts. The olivine gabbros (Samples TCH 23 and BG 28) have a convex upward pattern in their primitive mantle normalized diagram, and also clearly different from E− and N− MORB patterns (Fig. 11b). This further supports the results from the clinopyroxene. The Bikilal− Ghimbi parental basalt composition is identical to a typical intraplate− type tholeiite overall, except for the elevated Th to Nb contents (Fig. 11b). This may reflect a slight fluid metasomatism or an alteration of the clinopyroxene, as detected in the altered clinopyroxenes in Sample TCH 23 (Fig. 8). The olivine gabbros are apparently fresh, but traces of alteration/metasomatism may be present. However, the overall similarity between the estimated melt compositions and the recalculated clinopyroxene compositions strongly support an intraplate tholeiitic nature for the parental magma of the Bikilal− Ghimbi gabbro. The above evidence suggests that the Bikilal− Ghimbi gabbro was perhaps formed by the introduction of a man-

tle plume to generate intraplate−type tholeiitic magma during the Neoproterozoic time in a continental or oceanic setting. Alternatively, it could also represent an intrusion in a marginal ocean basin between Gondwana fragments during the Neoproterozoic time. This gabbro body was subsequently accreted to a suture zone, or a plume source magma intruded an accretionary suture. Reliable age dating will provide a more conclusive argument concerning the continental or oceanic setting, and will clarify the pre−, syn−, or post−tectonic nature of the intrusion. This is beyond the scope of this paper, and SHRIMP zircon age− dating is now in progress. The results will be presented in a separate paper. Crystallization conditions and magma replenishment Mathematical simulations of the crystallization sequence of the basalt magma using thermodynamic models have recently become available. The computer software programs that can be used include MELTS (Ghiorso and Sack, 1995) and COMAGMAT (Ariskin and Barmina, 2003). As MELTS does not accommodate pyroxene stability for the modeled intraplate−type tholeiitic magma, the COMAGMAT program was used to calculate the crystallization sequence. The starting melt composition was set as a typical intraplate tholeiitic basalt, as repre-

Petrogenesis of the Neoproterozoic Bikilal− Ghimbi gabbro

Figure 13. (a) Mineral mode simulation results for the fractional crystallization of the OIT using the COMAGMAT software package compared to the Bikilal− Ghimbi gabbroic intrusion data for fractional (Fc) and equilibrium (Ec) crystallization at 1 kbar. (b) Mineral chemistry simulation results for fractional crystallization of the OIT using the COMAGMAT software package compared to the gabbroic intrusion data for fractional (Fc) and equilibrium (Ec) crystallization at 1 kbar. The different symbols denote different pressures (1, 5, and 10 kbar), which can be reproduced from the initial tholeiite magma.

sented by the Hawaiian tholeiite (Wilson, 1989, Table 9.6, Kilauea and Mauna Loa tholeiites). The crystallization sequences differed according to changes in pressure and oxygen fugacity. The best solution was achieved with the conditions of: (1) low pressure (1 kbar); (2) high oxygen fugacity (~ QFM + 1); and (3) two contrasting crystallization conditions, fractional crystallization or equilibrium crystallization. The resulting model calculations reproduced the mineralogical modes, mineral compositions, and bulk composition of the Bikilal− Ghimbi gabbro (Figs. 12 and 13). The COMAGMAT modal compositions were 14−18% olivine, 22−35% clinopyroxene, and 54−58% plagioclase, which are similar to the observed values of the Bikilal− Ghimbi gabbro (Table 1). The calculated mineral compositions were Fo53−61 for olivine, Mg# = 75−70 for clinopyroxene, and An55−62 for plagioclase, all of which are also in good agreement with the observed compositions of the Bikilal− Ghimbi gabbro (Tables 3− 6). The calculated bulk solid composition is almost identical for TiO2, Al2O3, FeO, CaO, and

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Figure 14. Apatite saturation conditions for the Bikilal− Ghimbi gabbro. The source OIB with 1.00 wt% P2O5 was used in the COMAGMAT software simulations. Apatite saturation can occur at 1095 °C and with a degree of crystallization of 80−90. The precipitation temperature of apatite is lower than that of olivine, clinopyroxene, and plagioclase.

the alkali metals. However, some low MgO gabbros are not identical, and might have different parameters due to their evolved nature (e.g., temperature, pressure, and oxygen fugacity) (see Fig. 12). These results support our conclusion that the Bikilal− Ghimbi olivine gabbro crystallized from a composition that lay between a total fractional and an equilibrium crystallization. The key intensive variables of the crystallization conditions are a low pressure (1 kbar) and a temperature of 1100−1160 °C (Fig. 12f). Apatite does not host alkali metals, FeO, or MgO. Therefore, the Fe enrichment of the apatite−bearing subsets is not an effect of the apatite concentration, but may reflect the different activity of oxygen and water during the shallow level magma chamber fractionation rather than any fundamental difference in the chemistry of the parental magma (Wilson, 1989). As noted above, judging from the binary plot diagrams, the apatite crystallization may have been caused by a mixing between a low−Mg content gabbro from an evolved melt and a high−Mg precipitate from a mafic melt (Figs. 5 and 6). Such a high − MgO end member could be derived from a less− evolved magma that had a higher temperature (> 1160 °C; see Fig. 12). It is known that the SiO2 and P2O5 contents and the temperature are the controlling factors for apatite precipitation (Watson, 1979). The SiO2 content of the fractionating melt remained at ~ 50 wt% during the tholeiitic crystallization (COMAGMAT results not shown). Conse-

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B.W. Woldemichael and J.−I. Kimura

quently, the apatite saturation is a function of the P2O5 content of the melt and the temperature. The P2O5 content of the mafic melt increases with increasing fractionation (Fig. 14). From our model, P2O5 saturation occurs at ~ 1100 °C, and then apatite crystallization occurs at a very late stage of crystallization from an evolved melt. However, this ignores the role of the high−Mg high−P end member deduced from the major elements (Fig. 5). In contrast, if a high−temperature/high−MgO melt is forced to cool, then apatite saturation occurs even when the P2O5 content in the melt is low, due to the downward shift of the apatite saturation isotherm (see where P2O5 = ~ 1 wt% at 1000 °C, Fig. 14). This mechanism accounts for the mixing line observed in the apatite−bearing subsets. If an unfractionated basaltic melt is input into a low temperature evolved melt, then the crystallization of apatite, along with a high−MgO olivine, clinopyroxene, and perhaps a calcic plagioclase would take place from the basalt melt due to supercooling. The formation of an immiscible liquid is a possible origin of the large amount of apatite (> 20 modal percent) in the apatite−rich layers. Von Gruenwaldt (1994) proposed that an immiscible Fe−Ti− Ca−P liquid was periodically developed in the Bushveld complex, and suggested that the development of the mineralized zones at the bottom of the body is the result of the formation of an immiscible liquid by replenishment and magma mixing. Crystallization of apatite and Fe−Ti oxides in the Bikilal− Ghimbi gabbro is associated with Mg−rich mafic minerals that cannot be accomplished by an immiscible liquid alone. We believe that the supercooling of the replenished unfractionated basalt melt is crucial for the origin of the layering with a periodic formation of an immiscible liquid. The combination of fractional crystallization and magma replenishment has been reported from many layered intrusions (e.g., Clark, 2004; Zhou et. al, 2005), and evidence of supercooling of basaltic magmas in layered intrusions has also been documented (e.g., Ballhaus and Glikson, 1989; Tegner et al., 1993). CONCLUSIONS The Bikilal− Ghimbi gabbro consists of apatite−free olivine gabbro, hornblende gabbro, and hornblendite suites and their apatite−bearing equivalents. Despite the modal differences between the olivine and the hornblende gabbros, there is no systematic chemical difference in the major and trace element compositions, except for enrichment in the fluid −mobile elements in the hornblende− bearing suites. This suggests that the gabbros were generated from the same parental magma, unlike previous interpretations, where the hornblende gabbro suites and the olivine gabbros were separate intrusions. The horn-

blende−bearing suites contain uralitized hornblende and plagioclase with kink bands. These facies occur only at the perimeters of the gabbro body, and are structurally concordant with the host gneiss. Mineralogical constraints from the olivine and plagioclase suggest an intraplate−type tholeiitic composition. The estimation of the parental magma from the trace element compositions in the clinopyroxene and bulk rock also supports an intraplate tholeiitic nature of the parental magma of the gabbros. Therefore, we conclude that the Bikilal− Ghimbi gabbro formed by the introduction of a mantle plume to generate a tholeiitic magma during Neoproterozoic time. Although the mineralogical data suggest an intraplate setting, it cannot provide an unequivocal conclusion given the apparent pre−, syn−, or post−tectonic relationship of the intrusion with the surrounding gneissic terrain. Crystallization model calculations using a tholeiitic parental magma suggest that the gabbros crystallized from a basaltic magma, in which no interstitial melt remains. The apatite−bearing subsets are likely to have originated from the supercooling of the replenished basalt that precipitated both apatite and high−Mg mafic minerals. Our petrological and geochemical examination of this Neoproterozoic gabbro suggests that it is an intraplate−type tholeiite in composition. Isotope studies combined with a reliable zircon SHRIMP age− dating technique will be important for clarifying the tectonic settings of the gabbro, and this data will be presented in another publication. A full reappraisal of the other gabbro bodies in western Ethiopia is also necessary to reconstruct a reliable development history of the suture zone in the ANS. ACKNOWLEDGMENTS We greatly appreciate the support of the Geological Survey of Ethiopia for logistical assistance in the field, for provision of samples from the core and outcrop archives, and for granting extended leave to B.W.W. for this study. Dr. R.J. Stern of the University of Texas at Dallas, USA, provided thoughtful comments on an early version of the manuscript. This work was carried out at Shimane University with scholarship support from the Japanese Ministry of Education, Culture, Sport, Science and Technology to B.W.W. We also thank Drs. Y. Sawada and M. Akasaka of Shimane University, Japan for free access to the XRF and EPMA instruments and to Dr. BP Roser of Shimane University, Japan for valuable comments on the draft manuscript. The authors are also grateful to an anonymous reviewer and Dr. Sakae Sano of Ehime University for their valuable suggestions and critical comments regarding the manuscript.

Petrogenesis of the Neoproterozoic Bikilal− Ghimbi gabbro

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