SHRIMP U-Pb zircon geochronology of the Huai'an Complex: Constraints on Late Archean to Paleoproterozoic magmatic and metamorphic events in the Trans-North China Orogen
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Abstract:
The Huai9an Complex is situated in the northern segment of the Trans-North China Orogen (TNCO), a continent-continent collisional belt along which the discrete Archean Eastern and Western Blocks amalgamated to form the basement of the North China Craton. The complex consists of six distinct lithologic units: the Huai9an TTG gneisses, the Manjinggou high-pressure mafic granulites, the Khondalite Series, the Dongjiagou granitic gneiss, the Huai9an charnockite, and the Dapinggou K-feldspar granite. SHRIMP U--Pb geochronology, combined with Th and U data and cathodoluminescence (CL) imaging of zircons, enables resolution of magmatic and metamorphic events that can be directed towards understanding the late Archean to Paleoproterozoic history of the TNCO. CL images reveal the coexistence of magmatic and metamorphic zircons in most lithologies of the Huai9an Complex, of which the metamorphic zircons occur as either single grains or overgrowth rims surrounding and truncating oscillatory-zoned magmatic zircon cores. SHRIMP U--Pb analyses on magmatic zircons reveal that the tonalitic, trondhjemitic and granodioritic protoliths of the Huai9an TTG gneisses were emplaced at 2515 ± 20 Ma, 2499 ± 19 Ma and 2440 ± 26 Ma, respectively, much earlier than the emplacement of the Dongjiagou granitic gneiss dated at 2036 ± 16 Ma. However, their metamorphic zircons yield similar concordant ^207^Pb/^206^Pb ages of 1847 ± 17 Ma, 1842 ± 10 Ma and 1847 ± 11 Ma for the tonalitic, trondhjemitic and granodioritic gneisses, respectively, and 1839 ± 46 Ma for the Dongjia granitic gneiss. These ages demonstrate that the Huai9an Complex underwent a regional metamorphic event at ∼1850 Ma, which is further supported by a mean ^207^Pb/^206^Pb age of 1848 ± 19 Ma for metamorphic zircons in the Manjinggou high-pressure mafic granulite and 1849 ± 10 Ma and 1850 ± 17 Ma for igneous zircons in the anatectic Huai9an charnockite and Dapinggou garnet-bearing S-type granite, respectively. The timing of late Archean to Paleoproterozoic magmatism and regional metamorphism in the Huai9an Complex is in general agreement with recent SHRIMP zircon data for other metamorphic complexes in the TNCO. These data prove that the high-grade gneiss complexes were not the basement to the low-grade granite-greenstone terranes in the TNCO. Furthermore, the lithologies of the orogen are considered to have developed as a long-lived magmatic arc that was subsequently tectonically disrupted and juxtaposed during the collision of the Eastern and Western Blocks at ∼1.85 Ga, leading to final assembly of the North China Craton.Keywords:
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Pyroxene granulites and charnockites are associated with metasedimentary khondalite (garnet-sillimanite gneiss) and migmatites in the Eastern Ghats mobile belt. Various stages of in situ granulitization similar to those observed in southern India and Sri Lanka are present. Geochemically the granulite suite around Visakhapatnam is bimodal, with acid and mafic variants. There is an increase in the concentration and fractionation of the rare earth elements (REE) from pyroxene granulites to charnockites, and six Sm-Nd models ages () range from 2.86 to 2.35 Ga, with 2.1 Ga for a younger granite. The charnockites, a pyroxene granulite, and granite define a Pb/Pb isochron age of . U-Pb ages of zircons and monazites from a charnockite near Phulbani, Orissa, yield near concordant data with ages of and , respectively. The new isotopic data and the available ages from the northeastern part of the Eastern Ghats in Orissa suggest a major charnockite formation event between ca. 1170 and 950 Ma. This event is contemporaneous with that of Sri Lanka but much younger than the late Archean granulite event in southern India.
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Summary The charnockite series consisting of pyroxene granulites and charnockites are not formed by magmatic differentiation, although both units had their antecedents in igneous material. The solid-state recrystallization of pyroxene granulite and the liquid-state crystallization of charnockite are inferred from the field criteria. The pyroxene granulites with the associated paragneisses are formed from the original sedimentary-volcanic sequence under granulite facies of metamorphism. The magmatic charnockites emplaced into these rocks resulting in the development of intermediate varieties by the partial assimilation and incorporation of the pyroxene granulites. The stability of the minerals is not affected by this migmatization, which is therefore thought to have taken place under granulite facies conditions. The evolutionary trend of the charnockite series is one of retrogression from the earlier granulite facies to the later almandine-amphibolite facies, caused by the subsequent granite invasion.
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The Eastern Ghats Granulite Belt, India, with two major lithological associations: charnockites and meta sedimentary granulites, is characterized by polyphase deformation and complex, possibly multiple granulite events. Barring the cratonic margins in the north and west, two distinct crustal domains have been identified: the Eastern Ghats Province (EGP) and Ongole domain, separated by the Godavari graben. These domains also have distinct geochronological record of granulite event: in the EGP the first granulite event has been recorded as between 1.2 and 0.9 Ga; while in the Ongole domain the granulite event is recorded as 1.6 - 1.7 Ga. However, charnockite-massifs in both the domains, interpreted as product of deep crustal anatexis under granulite facies conditions, could provide a link in tectonic evolution of the EGB as a whole. LA-ICP-MS analysis of zircon spot ages of two charnockite massifs reveals vestiges of the1.6 Ga charnockite magmatism in the EGP as identical to that in the Ongole domain. Another charnockite massif in the EGP records concordant zircon spot age of 940 Ma, but single spot age of 990 Ma could indicate a prolonged UHT event. Thus magmatic charnockites of intracrustal melting origin could represent two granulite events, at ca. 1.6 and 1.0 Ga in the Eastern Ghats Belt. Also, accretionary orogenic processes of the Supercontinent Columbia might have encompassed the Eastern Ghats Belt with Australia, Antarctica and Laurentia.
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The Eastern Ghats granulite belt of India has traditionally been described as a Proterozoic mobile belt, with probable Archaean protoliths. However, recent findings suggest that synkinematic development of granulites took place in a compressional tectonic regime and that granulite facies metamorphism resulted from crustal thickening. The field, petrological and geochemical studies of a charnockite massif of tonalitic to trondhjemitic composition, and associated rocks, document granulite facies metamorphism and dehydration partial melting of basic rocks at lower crustal depths, with garnet granulite residues exposed as cognate xenoliths within the charnockite massif. The melting and generation of the charnockite suite under granulite facies conditions have been dated c . 3.0 Ga by Sm–Nd and Rb–Sr whole rock systematics and Pb–Pb zircon dating. Sm–Nd model dates between 3.4 and 3.5 Ga and negative epsilon values provide evidence of early Archaean continental crust in this high-grade terrain.
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