کد مقاله کد نشریه سال انتشار مقاله انگلیسی نسخه تمام متن
6428565 1634742 2015 15 صفحه PDF دانلود رایگان
عنوان انگلیسی مقاله ISI
Partial melting of thickened continental crust in central Tibet: Evidence from geochemistry and geochronology of Eocene adakitic rhyolites in the northern Qiangtang Terrane
موضوعات مرتبط
مهندسی و علوم پایه علوم زمین و سیارات علوم زمین و سیاره ای (عمومی)
پیش نمایش صفحه اول مقاله
Partial melting of thickened continental crust in central Tibet: Evidence from geochemistry and geochronology of Eocene adakitic rhyolites in the northern Qiangtang Terrane
چکیده انگلیسی


- The Suyingdi rhyolites erupted at 38.2 Ma and show geochemical characteristics of adakitic rocks.
- The rhyolites were most likely produced by partial melting of a thickened lower crust.
- The lower crust of central Tibet is heterogeneous.
- The low-velocity zones in the deep crust of central and northern Tibet were generated by crustal melting.

The composition of the deep crust is a key to understanding the formation of the low-velocity zone in the middle to lower crust of the Tibetan Plateau. The Suyingdi rhyolites exposed in the northern Qiangtang Terrane have high Sr (296-384 ppm) and low Y (5.81-7.93 ppm), with therefore high Sr/Y ratios (42-56), showing geochemical features of adakitic rocks. Zircon U-Pb dating yields an eruption age of 38.2±0.8 Ma (MSWD = 0.78). These adakitic rhyolites are high-K calc-alkaline in composition, displaying a weakly peraluminous character. They have low MgO content (0.20-0.70 wt.%) and Mg# values (24-39), as well as low Sc (2.25-2.76 ppm), Cr (8-14 ppm), Co (1.6-3.5 ppm) and Ni (2-3 ppm) concentrations. The rocks are LREE-enriched ((La/Yb)N = 50-62) and display weakly negative Eu anomalies (Eu/Eu⁎ = 0.82-0.95) and pronounced negative Nb and Ta anomalies. They have low initial (87Sr/86Sr)i ratios (0.707860 to 0.708342) and enriched Nd isotopic compositions with εNd(t) values ranging from −8.4 to −5.0, which are indistinguishable from those of Cenozoic potassic and ultra-potassic lavas exposed in northern Tibet. Their much higher SiO2 and lower Fe2O3 contents, yet similar MgO, Cr, Co, Ni, and Mg# values to the potassic and ultra-potassic lavas, however, indicate that the rhyolites are unlikely to have formed by fractional crystallization of these lavas. Because of their low Nb/Ta ratios and similar Sr-Nd isotopic compositions to granulite xenoliths within the Cenozoic potassic rocks, we infer that the Suyingdi adakitic rhyolites were most likely produced by partial melting of a thickened lower crust in the garnet stability field. The magma source is most likely dominated by granulite facies metabasalts and clay-poor metamorphosed sedimentary rocks which indicate that the lower crust of northern Tibet is heterogeneous. In combination with data from previously-reported peraluminous and metaluminous adakitic rocks in the same region, the age and petrogenesis of the Suyingdi adakitic rhyolites suggest that the low-velocity zone in the deep crust of central and northern Tibet was most likely the result of partial melting of thickened crust.

ناشر
Database: Elsevier - ScienceDirect (ساینس دایرکت)
Journal: Earth and Planetary Science Letters - Volume 414, 15 March 2015, Pages 30-44
نویسندگان
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