International Geologiical Congress - Oslo 2008

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ASI-01 Geodynamic evolution of Asia - Part 1

 

Neoproterozoic, Paleozoic and Mesozoic granitic magmatism in the Qinling orogen, China and their constraint on tectonic evolution

 

Tao Wang, Institute of Geology, Chinese Academy of Geological Sciences (China)
Xiaoxi Wang, Institute of Mineral Resources, Chinese Academy of Geological Sciences (China)
Zongqing Zhang, Institute of Geology, Chinese Academy of Geological Sciences (China)
Chenli Zhang, Department of Geology, Northwestern University (China)
Xinxiang Lu, Henan Institute of Geology Research (China)
Dawei Hong, Institute of Geology, Chinese Academy of Geological Sciences (China)
 

 

The Qinling orogen, one of the main orogens in East Asia, trends WNW-ESE across central China Voluminous intrusions occur in the orogen. They consist predominately of Neoproterozoic, Paleozoic, and Mesozoic. The Neoproterozoic pluton can be grouped as early (958 - 910 Ma) highly deformed and late (900-750 Ma) weakly plutons. Our new zircon ages of 958±7 Ma (TIMS) and 955 ±13Ma (SHRIMP) for the oldest highly deformed pluton and 929 ± 25 Ma (SHRIMP) for a weakly deformed granitic veins that intruded the pluton indicate that deformation must have taken place during 955-929 Ma. The early deformed granitoids are typical s-type (εNd(t) = -4.3 - -5.5). The late weakly deformed plutons show features of high-K calc-alkaline I-type and some are A-type. They show SIA-type evolution. This suggests Neoproterozoic syncollisional and post-collisional processes (or assemblage and break up), probably corresponding to the assemblage and breakup of Rodinia supercontinent. The continental assemblages appear to be later than Greenville orogeny (1300∼1000 Ma), but consist with the collision and continental-bock assemblages of the south China blocks.

The Paleozoic plutons(490 to 410 Ma) are mainly I-type (εNd(t) = 0 - +4) and a few of S-type (εNd(t) = -5 - 0). The plutons with ages of 490 - 450 Ma were regarded as results of subduction of the Qinling oceanic plate. The plutons with ages of 450 - 410 Ma in the core of the Qinling orogen were show eastward migration of their emplacement centers, corresponding the westward extrusion of the Qinling complex. That suggests progressive scissors-like collision from east to west.

The Mesozoic intrusions consist of early (Triassic) and late Mesozoic plutons. The Triassic intrusions have zircon age of 220 - 200 Ma. They show I-type or transition features of I- to A- type granites. Some of them are rapakivi-textured granitoids, a few of the known cases of rapakivi-textured granitoids in Mesozoic orogens. Mafic intrusions, enclaves (ca. 220 Ma, zircon ages) and lamprophyre dykes (219 ± 2 Ma, biotite 40Ar-39Ar), associate with the Mesozoic (rapakivi-textured) plutons, constituting bimodal magmatism. The exact timing of Mesozoic collision along the Qinling-Dabie orogenic belt is controversial. The Qinling lamprophyre dykes and the bimodal association indicate an post-collisional extensional tectonic setting at ca. 220 Ma, supporting the collision at ca. 240 Ma. The late Mesozoic intrusions have ages ranging from 160 to 120 Ma. They are characterized by small porphyry plutons or large pluton and regarded as post or non-orogenic.

The Neoproterozoic, Paleozoic and Mesozoic intrusion and their evolution natures reveal three important orogenic processes, and clearly demonstrate that the Qinling orogen is a composite orogen that underwent long-term and multi (at least three) orogenic processes, which is a few of the known cases of a complication orogens in the world.

 

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