Multiple Mesozoic magma processes formed the 240–185 Ma composite Weishan pluton, South China: evidence from geochronology, geochemistry, and Sr-Nd isotopes

Abstract

Mesozoic granitic intrusions are abundant in the South China Block (SCB). Understanding the generation of these granitic intrusions is important to better constrain tectonic regimes and mineralization processes. We studied the Weishan pluton, one of the largest Mesozoic composite granitic complexes in Hunan Province, to probe the relationship between multiple magma processes and tectonic events induced by plate interactions. The samples can be classified into three groups. Group 1 is highly fractionated and weakly peralumious, with flatter REE patterns and more negative Eu, Ba, Sr, P, and Ti anomalies compared with other groups. Group 2 is peralumnious-metaluminous while Group 3 is peraluminous, but with higher MgO, FeO, ε_Nd(t), and lower initial ⁸⁷Sr/⁸⁶Sr in Group 2 than in Group 3. Whole-rock and biotite compositions from all groups define a good linear trend, with Group 1 and Group 2 samples defining two mixing end-members. Single-grain mica Rb-Sr isochron and LA-ICP-MS zircon U-Pb ages show that Group 1 formed at ca. 240–230 Ma, Group 2 at 220–215 Ma, and Group 3 during two episodes at 210–205 Ma and 185 Ma. Integrated geochemical studies suggest that precursor magmas of the three groups were emplaced under oxidizing conditions at relatively low temperatures (<830°C). These magmas were mainly derived by partial melting of Palaeoproterozoic psammites but subsequently differentiated by fractional crystallization, crustal contamination, and remelting. Considering regional tectonothermal events, it is proposed that the oldest Group 1 granitic melts were derived by low-degree partial melting of thickened Palaeoproterozoic psammitic materials during prograde metamorphism due to the collision of the Indochina Block (ICB) and the SCB. The slightly younger Group 2 granitic melts were generated by further partial melting in response to stress relaxation in the post-collision stage and conductive heating from underplating mafic magma. The youngest Group 3 melts represent remobilized buoyant magma pulses supplied from a mushy Group 2 magma source accompanying regional reheating, possibly associated with the uplift of the SCB following its deep subduction during the Dabie orogeny and the subduction of the Palaeo-Pacific oceanic plate beneath the Eurasia plate, respectively.

Keywords:

• composite granite
• Indosinian
• Yanshanian
• South China
• Mesozoic

Acknowledgements

The authors thank Prof. X. Zhou for discussions and F. Wang and L. Chen for help with the sample EPMA analyses. Prof. Robert E. Zartman polished the manuscript and improved the English. Reviews by two anonymous referees and the comments of Prof. Robert Stern have greatly helped improve the paper. This is also a contribution by No. IS-1860 from GIGCAS.

Additional information

Funding

This study was supported jointly by the Ministry of Science and Technology of China [2012CB416703]; the National Natural Science Foundation of China [41090373], [41372005], and [41121002]; the State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, CAS [GIGIso-QD-12-04, Y436021A07DX]; and the 135 project of GIGCAS [Y234152001].