L.B. Terent'eva. 2. , I.K. Kozakov. 2. , A.B. Kotov. 2. 1 Institute .... Dergunov A.B., 61-94, Routledge Taylor and Francis Group,. London and New York, 2001. Wu F.
“HERCYNIAN” CRUST FORMATION IN THE CENTRAL ASIA OROGENIC BELT: GEOCHEMICAL, GEOCHRONOLOGICAL AND ND ISOTOPIC DATA
V.V. Yarmolyuk1, V.P. Kovach2, V.I. Kovalenko1, E.B. Salnikova2, L.B. Terent’eva2, I.K. Kozakov2, A.B. Kotov2 1
Institute of ore deposits geology, petrography, mineralogy and geochemistry, Russian Academy of Sciences, Staromonetnyi per., 35, Moscow 109017, Russian Federation 2 Institute of Precambrian Geology and Geochronology, Russian Academy of Sciences, Makarova emb., 2, St-Petersburg 199034, Russian Federation
Geochronological and Sr-Nd isotopic data obtained during last 15 years for the Phanerozoic granitoids of the Central Asia, Cordillera of North America and New England and Lachan Fold Belt in south-eastern Australia [Jahn et al., 2000a; Jahn et al., 2000b; Kovalenko et al., 1996; Kovalenko et al., 1999; Kovalenko et al., 2004; Patchett, 2004; Wu et al., 2000] demonstrate that most of them are characterised by low initial Sr ratios and positive HNd(T) values. It cardinally differs from “classic” Caledonian and Hercynian granitoids of Europe [Jahn et al., 2000a; Jahn et al., 2000b] and suggests significant Phanerozoic addition of juvenile continental crust. At the same time such questions as episodes, mechanisms of juvenile crust production and their sources are still issue of debate. Recently [Kovalenko et al., 2004] “Caledonian” (Late Neoproterozoic – Early Paleozoic) crust-forming event in the Central Asia Orogenic Belt (CAOB) has been identified on the basis of geological, geochronological and Nd isotopic evidences. However, “Hercynian” (Ordovician-early Carboniferous) structures of the CAOB still poorly investigated. We present new geochemical, geochronological and Nd isotopic data for island arc complexes and granitoids for the Hercynian belt of the South Mongolia in order to further improve our understanding of the mechanisms and processes of crust formation and evolution in the Phanerozoic. The “Hercynian” belt of the South Mongolia borders the Caledonian mobile belt and from the south transit to the Hercynian structures of the north-west China and then Kazakhstan and bounded by South Gobi microcontinent and Indo-Sinides of Inner Mongolia from the south. The “Hercynides” of the South Mongolia include the Gobi-AltaiSukhe-Bator zone, the Tsel metamorphic belt, the Edrengin and the Transaltai zone [Ruzhentsev, 2001]. The Gobi-Altai-Sukhe-Bator zone is mostly terrigeneous and characterised by strong tectonic layering and combination of the Caledonian and Hercynian complexes. The Edrengin zone is composed mainly by mature island arc complexes (D2-3) whereas Transaltai zone consist of ophiolites, volcanic and volcano-sedimentary sequences (S-D1). Granitoids are widely distributed in the studied area and formed from ca 350 to 120 Ma. 282
According to geological data [Ruzhentsev, 2001] ophiolite complexes of the Transaltai zone were formed at S-D1 and represented by pillow and massive basalts, dykes of the sheet-complex and rocks of the layered sequence forming separate allochtonous napes (slabs). Rocks of the ophiolite associate with thick deposits of siliceous aleurolites and argillites with subordinate sandstones, gravelites and conglomerates. Basalts and basaltic andesites from the Nomin-Gobi ridge of the Transaltai zone are characterised by LREE-enriched REE patterns (LaN/YbN = 1.5-6.7), pronounced negative Nb-Ta anomalies (La/Nb = 1.6-5.4) typical for intra-oceanic island arc setting. They have high positive but lower that in DM İNd(T) values from +6.1 to +6.9 and were formed from juvenile mantle sources with addition of long-lived crustal material into subduction zones. Associated siliceous aleurolites show similar trace-element signatures but significantly lower values İNd(T) = +3.1 to +3.8 and Nd model ages TNd(DM) = 1.0 Ga suggesting significant input of the old crustal sources. Volcanics of the Dzoilen complex of the Transaltai zone form two association: 1) low- to middle-Ti (ca 1.0-1.4 wt% TiO2) basalts and basaltic andesites with LREEenrichment (LaN/YbN = 3.1-7.2), negative Nb-Ta anomalies (La/Nb = 2.6-3.6, Th/Ta = 5.8-7.5) and fractionated HREE pattern (GdN/YbN = 1.5-2.0); 2) middle-Ti (ca 1.5-1.6 wt% TiO2) basalts with flat REE distribution (LaN/YbN = 1.0-1.1) and depletion in Th, Nb and Ta (La/Nb = 2.1-2.7, Th/Ta = 2.2-2.7). Geochemical data suggest formation of the first association in the island arc and second – in the back-arc tectonic setting. However both group of volcanics show similar İNd(T) from +6.9 to +7.7 and late Neoproterozoic Nd model ages of 0.54-0.56 Ga. Siliceous aleurolites are characterised by trace-element signatures similar to group 1 volcanics whereas black shales exhibit flat trace-element distribution with positive Nb-Ta anomalies similar to basalts of the oceanic plateaus. Thus sediments of the Dzoilen complex have different provenance sources. They yield positive İNd(T) values of+6.3 and +5.9 respectively. The Edrengin zone is composed by mainly mature middle – late Devonian island arc complexes with andesites, dacites and rhyolites associated with volcanomictic gravelites and conglomerates. Volcanics are characterised by fractionated trace-element distribution with prominent negative Nb-Ta (La/Nb = 3.1-5.2, Th/Ta = 7.6-12.8), Zr-Hf and Ti anomalies, enrichment by LREE (LaN/YbN = 3.7-5.3) and fractionated HREE patterns (GdN/YbN = 1.4-1.6). They yield high positive İNd(T) values from +6.4 to 6.9 and TNd(DM) ca 0.7 Ga. Parental melts of basalts, andesites and dacites of the Edrengin zone were formed from juvenile long-term depleted sources with contribution of the old crustal material in the oceanic island arc setting. “Hercynian” paleooceanic complexes have been accreted to the “Caledonian” continent of the CAOB at ca 385-360 Ma ago. Granitoids of the Transaltai, Edrengin and Gobi-Altai-Sukhe-Bator zones were formed at 348±2, 328±1, 302±2, 290±5 and 274±5 Ma ago in response to subduction processes in the Indosinian mobile belt and intraplate activity. Granitoids of the Transaltai zone are characterised by high positive İNd(T) values from +5.5 to +7.2 and TNd(DM) = 0.48-0.58 Ga (two-stage Nd model ages TNd(DM-2st) = 0.50-0.60 Ga). Granitoids of the Edrengin zone have ssimilar Nd isotopic characteristics: İNd(T) = +5.9 to +6.6 and TNd(DM) = 0.54-0.57 Ga (TNd(DM-2st) = 0.56-0.59 Ga). On the 283
“İNd – Age” diagram granitoids of the Transaltai and Edrengin zones plot in the Nd isotope evolution field of the island arc – back arc volcanics that suggest derivation of the granitoid melts from mainly “Hercynian” island arc sources. Opposite to Transaltai and Edrengin zones granitoids of the Gobi-Altai-SukheBator zone (Dzhinset subzone) have İNd(T) values from +3.3 to +4.6 and Nd model ages of 0.63-0.79 Ga (TNd(DM-2st) = 0.70-0.78 Ga) similar but slightly higher to those for granitoids of the “Caledonian” Lake zone and Mongolian Altai. On the “İNd – Age” diagram they plot below the Nd isotope evolution fields of “Caledonian” and “Hercynian” juvenile crusts that suggest Dzhinset subzone granitoids were probably formed by partial melting of short-lived juvenile and long-lived crustal sources. Obtained geological, geochronological and Nd isotopic data suggest that “Hercynian” continental crust of the CAOB in the South Mongolia had been formed by transformation of oceanic crust via subduction to juvenile island arc and continental crust during Silurian –Devonian time and indicate distinct – “Hercynian” – crust-forming event in the CAOB. Ca 350-270 Ma granitoids were mainly formed by reworking of juvenile “Hercynian” crust.
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