U-Pb zircon, geochemical and Sr-Nd-Hf isotopic constraints on the age and origin of Early Palaeozoic I-type granite from the Tengchong-Baoshan Block, Western Yunnan Province, SW China

摘要

Herein we present new U-Pb zircon ages, whole-rock geochemical data and Nd-Sr-Hf isotopic data for an Early Palaeozoic monzogranite batholith from the Tengchong-Baoshan Block, Western Yunnan Province, China. Mineralogical and geochemical features suggest that this granitoid is a high-K, calc-alkaline, I-type granite. SHRIMP and laser ablation ICP-MS (LA-1CP-MS) analysis of zircon yields ages of between 499 ± 5 Ma and 502 ± 5 Ma, for three samples from the batholith. The monzogranite is characterised by high initial ~(87)Sr/~(86)Sr (0.7132-0.7144), negative ε_(Nd)(t) (-9.7 to -9.40) and ε_(Hf(t) (-10 to -13.1), and is interpreted to derive from remelting of pre-existing Palaeoproterozoic, high-K, metabasaltic rocks of the upper crust. The granitoid magma underwent extensive fractional crystallisation of biotite + hornblende, ilmenite, titanite, K-feldspar and plagioclase during emplacement. The crystallisation temperature of the magma lies in the range 633-733℃, however, there is no evidence to suggest crustal assimilation occurred during its ascent. Like the ～500 Ma, I-type granite of this study, there occur numerous granitoid rocks of Early Palaeozoic age (490-470 Ma) in adjacent regions across the entire Tengchong-Baoshan Block (Chen et al., 2004, 2005; Song et al., 2007). This episode of plutonism is coeval with the widespread granitoid magmatism found throughout the Indian Plate and the Himalayan Oro-genic Belt that are both subordinate parts of the ancient, Gondwana supercontinent. We infer, therefore, that the Tengchong-Baoshan Block may also have formed part of Gondwana, and that it separated from this supercontinent along with other crustal blocks during the Late Palaeozoic. Moreover, based on the findings of this study, we document the occurrence of arc-related magmatism in the Tengchong-Baoshan Block during the late Palaeoproterozoic.