Metamorphosed Basic-Calcareous-Siliceous Rock Association in Yangtze Miaowan Recorded Neoproterozoic Arc-Continent Collisional Event

Metamorphosed basic-calcareous-siliceous rock association recorded in Miaowan Ophiolite suite from the Northern Yangtze Craton. The garnet-amphibolite-gneiss, banded quartzite-marble are kept in the amphibolites as lens and layers.

Based on petrographic studies, the garnet-amphibolite-gneiss experienced three episodes of metamorphism: (M1) Early prograde metamorphism, represented by the hornblende, plagioclase¹ and quartz inclusion kept in the garnet; (M2) Peak metamorphism, displayed by the garnet and the matrix, as rock combination of garnet, hornblende, plagioclase² and quartz; (M3) retrograde metamorphism, recorded by the rim of the garnet and the retrograded matrix, as the mineral combination of quartz, plagioclase³ and chlorite.

From the mineral chemistry analysis, the ferric content from the core of the garnet is as same as the matrix, as well as the magnesium and ferric content from the garnet-amphibolite-gneiss (the content of MgO and Fe₂O₃ are 5.64 % and 13.5 % separately) are close to those content from the averaged amphibolite (the content of MgO and Fe₂O₃ are 7.81 % and 13.9 % separately).

According to the protolith reconstruction with whole-rock major and trace element, the protolith of the amphibolites were MORB, the protolith of the garnet-amphibolite-gneiss were greywacke, while the protolith of the quartztite-marble are siliceous limestone. The metamorphosed basic-calcareous-siliceous rock association indicates that the combination of basalt (oceanic crust), oceanic crust with terrigenous sediments, siliceous limestone experienced high-amphibolite grade metamorphism.

The lower interception age of zircons from the garnet-amphibolite-gneiss suggests that the high-amphibolite grade metamorphic event happened at 1000 Ma. Besides, the inherited zircon cores of core-rim mircostructure indicate their protolith of the sediments mainly formed at 1150 Ma, while older crustal relic should be also recorded by few concordant zircon age of 1380 1680 Ma and interception age of 2900 Ma. The protolith of the metasedimentary rocks have affinity to the Neoproterozoic Shennongjia arc due to the similar age distribution.

Hence, the metamorphosed basic-calcareous-siliceous rock association in Kongling Miaowan recorded Neoproterozoic arc-continent collisonal event.