Unraveling the La Campana gabbro cumulate emplacement conditions and host-rock metamorfism, Coastal Range, Central Chile Pablo G. Molina1,2*, M.F. Soto1,3, Miguel A. Parada1,2 and Francisco Gutiérrez1,2,4
(1) Departamento de Geología, Facultad de Ciencias Físicas y Matemáticas, Universidad de Chile, Plaza Ercilla 803, Santiago, Chile. (2) Centro de Excelencia en Geotermia de los Andes (CEGA), Santiago, Chile. (3) IESE, University of Auckland, 58 Symmond St, Floor 6, Auckland, New Zealand. (4) Advanced Mining Technology Center, Facultad de Ciencias Físicas y Matemáticas, Universidad de Chile, Santiago 8370451, Chile.
D. WHOLE ROCK, MINERAL CHEMISTRY AND Ti-in-Hbl THERMOMETRY
*E-mail:
[email protected]
Whole rock SiO2 content vary from 47 to 53 wt.%, whereas #Mg is between 0.249 and 0.335. Chondrite normalized REE patterns are shown in Fig. 3. They show flat to steep (high LREE contents) enriched patterns, with variable Eu anomalies (Eu/Eu*=Eu/√GdxSm) that are between 0.83 and 1.37, directly correlated to whole rock SiO2 content. Plagioclase composition vary from An83 to An81 in cores, to An46 to An58 in rims. Clinopyroxene, commonly displaying ilmenite inclusions, magnesio-hornblende (lesser iron rich hornblende), magnetite and biotite, correspond to the intercumulus phases. Ti-in-Hbl thermometry (R3) of intercumulus hornblendes average ~570ºC.
A. INTRODUCTION
Sample/Chondrite 10 100
As previously described by Darwin in 1840 and later by Brüggen (1950), the La Campana gabbro crops out as a singular outstanding Lower Cretaceous (40Ar-39Ar in plagioclase age of 130±1.5 Ma, R4) gabbro-dioritic body in the Coastal Range of Central Chile (reaching approximately 1850 m.a.s.l.). It intrudes a Lower Cretaceous volcanosedimentary succession deposited in a subsiding basin (dipping to ca. 30º-40ºE), consisting on: the Horqueta Fm. (intermediate to acid lavas and continental sandstones), the Lo Prado Fm. (ryholites, felsic tuffs and limestones), and the Veta Negra Fm. (porphyry basalts and basaltic andesites). The eastern margin of this intrusive body is in contact with the Upper Cretaceous Caleu Pluton. This intrusion generates important contact metamorphism haloes (300-1000 meters from the intrusive) that overprint low-grade burial non-deformative sub-greenschist metamorphic facies (R2), and develops copper and silver-bearing skarn-type deposits, reaching Hbl hornfels facies near its contact.
wt.% SiO2 47-48 48-49 49-50 50-51 51-52 52-53
B. CUMULATE TEXTURES
La
The coarse grained gabbros display orthocumulate and adcumulate textures (following the nomenclature of R6). Large euhedral to subhedral plagioclase is seen as early protoeutectic crystals with thin to thick post-cumulate xenomorphic overgrowths. Scarse peritectic anhedral opaques (magnetite and ilmenite) and late near-eutectic amphiboles fill intercumulus space (Fig. 2).
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
Figure 3. Chondrite normalized (R1) REE patterns for whole rock geochemical analyses of the La Campana gabbro cumulate.
E. CONTACT METAMORPHISM CONDITIONS
The metamorphic conditions reached were estimated by the study of two near-contact (less than 200 meters from the intrusive) country rock samples: a Hbl-phyllite hornfels (Lo Prado Fm.) and a nodular hornblende-ocoite (Veta Negra Fm.). For this purpose, PTX pseudosections were modeled with the software PerpleX v. 6.6.6 for the system MnNCKFMASHTO for whole rock compositions.
C. ISOTOPES
in
880 390
1660 1270
880 .61
p Am g in
-0 .57
0
390
0
0.5 Ttn
#M
400
500
600
Temperature (o C)
700
8
0.4
0
0.5
Gr
0.27 An
100 300
Chl out
#Mg in biotite
ut
0.710
Pressure (bar)
0.708
Titanite
0.706 87Sr/86Sr
1660
chl out
0.704
2050
2050
1270
Hbl-phyllite hornfels #Mg in amphibole
0.23 An
Late Paleozoic granitoids -5 0.702
2440
Jurassic volcanic rocks
b 2440
Ocoite hornfels
Vesuvianite in
Jurassic 0 granitoids
a
Pressure (bar)
εNd
Opx in
Veta Negra Fm. (c. 119 Ma) La Campana gabbro
5
to ut
Caleu Pluton (c. 95 Ma)
to
10
The isopleth intersection of stable metamorphic phases in paragenesis (plagioclase and amphibole in the ocoite, biotite and amphibole in the Hbl-phyllite hornfels, Fig. 4) give good estimates for peak metamorphic conditions of 0.2-0.7 ±1 kbar and 770ºC (±10%) for the Hblphyllite, and 0.2-0.3 ±1 kbar and 750ºC (±10%) for the ocoite. These pressures can be interpreted as an equivalent depth of 1 to 2 kms for contact metamorphism. There is also a significant low temperature metamorphic overprint where chlorite, titanite and other retrograde phases appear (Fig. 4a). This metastable phases could be product of a second pulse of magmatism in the area such as the one given by the emplacement of the Caleu Pluton (c. 90-110 Ma, R5).
Gr
Pacific N-MORB
Peak P-T conditions
Figure 1. Whole rock Sr-Nd isotopes stand for eNd values between 3.55 and 4.48, and 87Sr/86Sr between 0.703968 and 0.704265. A CHUR present value of (143Nd/144Nd)=0.512638 is considered. Red field indicates the La Campana gabbro analyses made in this work. Figure modified from R5).
800
100 300
400
500
600
700
8
0.4
800
Temperature ( oC)
Figure 4. Simplified pseudosections for the ocoite hornfels and the Hbl-phyllite hornfels, (a) and (b) respectively. Retrograde chlorite, titanite and vesubianite apparition postdate peak metamorphic assemblages in ocoite hornfels. Gray circle/ellipse represents peak metamorphic conditions after intersect certain isopleths (compositions obtained from EMPA analyses), in each sample.
F. DISCUSSION AND CONCLUSIONS
-Taking into account the textural and whole rock geochemical data we believe that crystal settling and/or residual liquid migration, fractionating early stage plagioclase crystals from the melt, are the main factors that controlled the La Campana gabbro emplacement and its differentiation. -A shallow (1 to 2 km. depth) peak metamorphic event should correspond to conditions reached at the late-magmatic stage of the development of the gabbro cumulate crystal pile, given the low metamorphic temperatures recorded (between 750-770°C) and the proximity of these samples to the intrusion. -Low temperature (c. 570°C) near-solidus hornblende crystallization must have growth from higly differentiated interstitial melts. The formation of these crystals should postdate the peak metamorphic event recorded in the host rock, depending on the thermal conductivity of the adjacent successions (Veta Negra Fm. and Lo Prado Fm.). -The isotopic ratios exposed here (Fig. 1), suggests that a genetic relationship between the La Campana gabbro and surrounding rocks is plausible.
Acknowledgments
This study is financed by the CONICYT-FONDAP project 15090013, Andean Geothermal Centre of Excellence (CEGA); and the Msc. CONICYT grant. Many thanks to Francisco Hervé and Enrique Tidy who provided helpful guidence; Thomas Theye, Hugo Quinteros and Iván Gómez, for EMPA and sampling assistance, respectively. Figure 2. Selected microscopic images representing typical textures and modal abundances found in the La Campana gabbro. Schematic textures at the right side of each picture with gray, black and white colors representing proto-eutectic, peritectic and near eutectic phases, respectively. (a) Orthocumulate texture in which protoeutectic plagioclase is surrounded by peritectic opaques and late intercumulus amphibole. Here plagioclase do not develop post-cumulate overgrowths. (b) Adcumulate texture evidencing fairly well preserved early automorphic plagioclases with thick postcumulate overgrowths. Intercumulus phases are nearly absent.
References
R1. Anders E., Grevesse N., 1989. Abundances of the elements: Meteoric and solar. Geochimica et Cosmochimica Acta. V 53, p. 197-214. R2. Levi B., 1969. Burial metamorphism of a Cretaceous volcanic sequence west from Santiago, Chile. Contributions to Mineralogy and Petrology. V 24, p. 30-49. R3. Otten Max,1984. The origin of brown hornblende in the Artjället gabbro and dolerites. Contributions to Mineralogy and Petrology. V 86,p. 189-199. R4. Parada M.A., Larrondo P., 1999. Thermocronology of the Lower Cretaceous Caleu pluton in the Coast Range of central Chile: tectono-stratigraphic implications. Fourth ISAG, Göttingen, Germany. Expanded abstract, p. 563-566. R5. Parada M.A., Féraud G., Fuentes F., Aguirre L., Morata D. and Larrondo P., 2005. Ages and cooling history of the Early Cretaceous Caleu Pluton: testominy of a switch from a rifted to a compressional continental margin in central Chile. Journal of the Geological Society, London. V 162, 205. p. 273-187. R6. Wager L.R. , Brown G. M., Wadsworth W. J., 1960. Types of Igneous Cumulates. Journal of Petrology. V 1, p. 73-85.
