Geological Implications Of Triassic Magmatism In Diancangshan And Chongshan Areas,western Yunnan

The Sanjiang orogenic belt in southeastern Tibetan Plateau belongs to the Tethys tectonic domain.The Ailaoshan and Changning-Menglian metamorphic belts along the Indochina block preserve a series of evolutionary history of the Paleo-Tethys Ocean.However,the tectono-magmatic evolution of the post-closure of the Paleo-Tethys Ocean is not clear.Besides,whether the structural nature of the Indosinian ophiolite in the Ailaoshan belt is a branch ocean or a back-arc basin remains controversial.The coeval early Mesozoic magmatic activities in Diancangshan and Chongshan areas provide a window to solve the above problems.In this paper,petrology,whole rock geochemistry,and zircon U-Pb-Hf isotopes are reported for a series of early Mesozoic diorite-quartz diorite-granitic rocks in the Diancangshan-Ailaoshan and Chongshan metamorphic belts.These data are used to constrain the formation age of these magmatic rocks,and deduce their magmatic origin,and establish the relationships of the tectonic settings of these two belts,which to further discuss the tectonic evolution of the Paleo-Tethys Ocean closure and the nature of the Indosinian ophiolite in the Ailaoshan belt(branch ocean or back-arc basin).Zircon U-Pb dating shows that the Diancangshan diorites and monzogranites were formed at 237 Ma and 232-228 Ma,respectively.The Chongshan quartz diorites were generated at233-228 Ma.Apart from sample CS1702 with formation age of 240 Ma,the other Chongshan monzogranites show ages from 226 to 221 Ma.The Diancangshan diorites show calc-alkaline affinities with low Si O₂,high Mg O,enrichment of LILEs,and depletion of Nb,Ta,Ti.They were derived from metasomatized mantle wedge and experienced fractional crystallization of clinopyroxene.The Chongshan quartz diorites display relatively low Si O₂,and high Mg O,Mg#,Cr,and Ni contents,together with their depleted?_Hf(t)values,indicating a mantle source.The presence of older inherited/xenocrystic zircons suggests crustal contamination of these rocks.The monzogranites of the Diancangshan and Chongshan areas are characterized by high Si O₂,K₂O/Na₂O and A/CNK and low Mg O contents with primary muscovite preserved within some rocks.They are peraluminous and derived from intracrustal reworking.The presence of inherited/xenocrystic zircons,and wide ranges of?_Hf(t)and Ca O/Na₂O demonstrate a mix source.These early Triassic diorites,quartz diorites,and monzogranites in the Diancangshan and Chongshan areas were generated during slab breakoff.The upwelling of hot asthenosphere induced partial melting of early metasomatized mantle,generating basaltic-dioritic magmas.Continuing underplate of these mantle magmas provided much hot to melt the sedimentary rocks and juvenile crust,gave birth to our granitic rocks.These Indosinian magmatic records show that the rock assemblages,magmatic petrogenesis,and tectonic settings are comparable between the Diancangshan and Chongshan areas.These records,together with previous comparison of a large amount of magmatic and sedimentary rocks related to the Paleo-Tethys Ocean on both sides of the Indochina block,we prefer that the Indosinian ophiolite in the Ailaoshan belt could be more plausible to represent a late Paleozoic to early Mesozoic Paleo-Tethys branch oceanic basin rather than a back-arc basin of the main ocean basin.The Paleo-Tethys main ocean and the branch ocean were closed at 237 Ma and 247 Ma,respectively,and then the asthenosphere upwelling was induced by slab breakoff and gave birth to a variety of magmatic rocks mentioned above.