Neoarchaean felsic volcanic rocks from the Shimoga greenstone belt, Dharwar Craton, India: Geochemical fingerprints of crustal growth at an active continental margin
Authors:
Manikyamba et al
Abstract:
The felsic volcanic rocks of Neoarchaean Shimoga greenstone terrane of western Dharwar Craton in India are dominantly represented by rhyolites occurring at stratigraphically upper horizons. The Shimoga rhyolites are associated with conglomerates, quartzites, argillites, limestones, cherts, basalts and intermediate volcanic rocks clearly suggesting an accretionary package. The rhyolites of Daginkatte and Shikaripura areas are potassic, with porphyritic alkali feldspar and quartz as essential minerals and chlorite, biotite and opaques as accessory phases. Geochemically, the rocks show enrichment in LILE and depletion in HFSE relative to primitive mantle values, with negative Nb-Ta, Zr-Hf anomalies and positive Th anomalies. These features of the Shimoga rhyolites compare well with the geochemical characteristics of magmas generated in subduction-related tectonic settings. Their alkaline compositions, intermediate to low HFSE abundances, moderate to high Zr/Y values (1.5–8.3), with La/Ybn (2–28), pronounced negative Eu anomalies, and variable LREE/HREE fractionation trends resemble the FI and FII rhyolites of Wabigoon and Uchi belts of Superior Province, Canada. The Shimoga rhyolites are interpreted to be products of melting of thick basaltic crust metamorphosed to amphibolite/eclogite grade, with garnet- and amphibole-bearing mantle residue. The rhyolites show prominent negative Eu and Ti anomalies, moderate to strong LREE fractionation, flat to mildly fractionated HREE patterns and are geochemically analogous to Type 1 and Type 3 rhyolites of Superior Province, Canada suggesting their derivation from intracrustal melting and fractional crystallization of basaltic liquids with prominent contribution from mantle wedge and slab components. Our data suggest the contribution of Neoarchaean active continental margin processes for the growth and evolution of continental crust in the western Dharwar Craton.
Multiple mafic magmatic and high-grade metamorphic events revealed by zircons from meta-mafic rocks in the Daqingshan−Wulashan Complex of the Khondalite Belt, North China Craton
Authors:
Liu et al
Abstract:
Meta-mafic rocks in the Daqingshan–Wulashan Complex, in the central segment of the Paleoproterozoic Khondalite Belt of the North China Craton, are composed predominantly of garnet-bearing mafic granulites, mafic granulites, and amphibolites. They occur mainly as irregular lenses and as a set of deformed dyke/sill swarms within dioritic–granitic orthogneisses and Al-rich paragneisses. An integrated study involving laser Raman and scanning electron microscope analyses of mineral inclusions, cathodoluminescence imaging, and LA–ICP–MS U–Pb dating of zircons, shows that magmatic zircons in these meta-mafic rocks record four age groups of 2500–2450, 2300–2100, 1970–1930, and ca. 1850 Ma, indicating multiple mafic magmatic events occurred within the Daqingshan–Wulashan Complex. In contrast, four groups of metamorphic zircons with ages of ca. 2450, ca. 1950, ca. 1900, and ca. 1850 Ma are present within these meta-mafic rocks. Zircons of the first (oldest) group contain medium- to low-pressure (MLP) granulite facies mineral inclusions of orthopyroxene + clinopyoxene + amphibole + plagioclase + quartz, and yield ages of ca. 2450 Ma, representing the timing of the early MLP granulite facies metamorphic event within the Daqingshan–Wulashan reworked basement complex. In contrast, zircons of the second group are free of mineral inclusions, and yield ages of ca. 1950 Ma, which is considered to be the timing of the regional high-grade metamorphic and structural event probably caused by the collision of the Yinshan and Ordos Blocks. Zircons of the third group contain MLP granulite facies mineral inclusions of orthopyroxene + clinopyoxene + amphibole + plagioclase + quartz, similar to the zircons of the first group, but yield younger ages of ca. 1900 Ma, interpreted as the timing of the regional MLP granulite facies metamorphism that most likely occurred during the early stage of post-orogenic exhumation of the Daqingshan–Wulashan Complex. Zircons of the fourth group contain MLP amphibolite facies mineral inclusions of clinopyoxene + amphibole + plagioclase + quartz, and yield the youngest ages of ca. 1850 Ma, which probably represent the timing of the regional amphibolite facies metamorphism during the late stage of post-orogenic exhumation of the Daqingshan–Wulashan Complex. Combined with previous geological data of the Khondalite Belt, it is speculated that the multiple mafic magmatism and contemporaneous high-grade metamorphism of the Daqingshan–Wulashan Complex from the Khondalite Belt were most likely generated by a complete orogenic process involving early subduction–collision and late exhumation in an active continental margin arc
