| Partial melting during post-collisional stage is a significant process to the evolution of an orogen.On one hand,melt activity can strongly influence the thermal and rheological behavior of orogenic crust,resulting in the orogenic collapse.Thus,it is tightly linked to the orogenic evolution.On the other hand,partial melting would trigger the migration and re-distribution of elements in crust.Hence,it is thus important for study intracrustal differentiation.Migmatites are the products of partial melting,and can thus provide important constraints on melting processes in orogens.Pervasive migmatites have been reported in the Tongbai and Dabie orogens,which provide a key target for studying the melt processes in orogens.In this study,we carried out a comprehensive study of petrology,high-precision zircon U-Pb geochronology,trace element,and Hf-O isotopic compositions,as well as whole-rock elements and Sr-Nd isotopes for migmatites from the Tongbai-Dabie orogenic belt.The results not only provide direct evidences for the timing,reaction,source of melt in the Tongbai-Dabie orogenic belt,but also shed light on compositional variation.This study brings a new insight into the spatio-temporal distributions of melt activities during post-collisional mountain-root collapse stage.The main achievements are listed as follows: 1.We carried out a combined study of cathodoluminescence(CL)imaging,U-PbHf-O isotopes and trace elements of zircons from migmatites in the Tongbai orogen.In these migmatites,most zircon grains present as overgrowth rims around the inherited cores,while some occur as new,weak zoned or unzoned crystals.Some zircon cores from melanosome 10TB29 show oscillatory or weak zoning,high Th/U ratios(0.20?1.31),variable and discordant U-Pb ages from 2443.4 ± 6.7 to 374.6 ± 16.9 Ma,and variable ?18O values(5.04 ± 0.27‰?10.95 ± 0.30‰),indicating their inherited origin and suffered subsequently metamorphic alteration.Nebulously zoned/unzoned cores of melanosome 10TB29 with low Th/U ratios(0.005?0.018)are also observed in this sample,suggesting they are metamorphic genesis.They exhibit flat MREE-HREE patterns with insignificant Eu anomalies,low formation temperatures of 654 °C,and yield U-Pb ages of 228 ± 3 Ma,indicting they formed under eclogite-facies conditions.The overgrowth rims and new grown grains show euhedral shapes,low CL luminescence,high Th/U ratios,clear negative Eu anomalies and steep HREE patterns,suggesting their formation during partial melting.They yield U-Pb ages from 139 ± 2 to 132 ± 1 Ma.Combined with previous studies,we suggest that there exists a prolonged process of partial melting in the Tongbai Orogen.The anatectic zircons in leucosomes 10TB30 show much lower ?18O(5.79 ± 0.23‰ vs.8.94 ± 0.16‰)and higher 176Hf/177 Hf ratios(0.282302?0.282648 vs.0.281728?0.282504)than the inherited cores in adjacent melanosome 10TB29,suggesting that the leucosomes were externally sourced.2.We performed an integrated petrologic,geochemical,zircon U-Pb-Hf-O isotopic and trace element study of migmatites from the Dabie orogen.Hornblende-plagioclase thermo-barometry indicates that the migmatites formed at P-T conditions of 3.5?5.5 kbar and 737?790 °C,suggesting partial melting of a biotite + plagioclase + quartz-bearing protolith under water-fluxed conditions.These geochemical features are listed as follows:(1)they are enrichment in Ba,Rb,depletion in Ti,Nb and P,variable Nb/Ta(2.00?15.3)and Zr/Hf(17.9?103)ratios,and low Rb/Sr ratios(0.01?0.13);(2)most of them have upconcave MREE-HREE patterns,marked positive Eu and Nd anomalies;(4)They have a positive relationship between LREE contents and Nd/Nd* ratios,and HREE and Zr contents.These data indicate that most of melt experienced crystal accumulation of plagioclase,dissolution of zircon and monazite,and fractional crystallization or stable residual of amphibole in various degrees.The formation timing of migmatites from Ta'erhe and Fenghuangguan areas are 127 ± 2 Ma and 136 ± 3?131 ± 1 Ma,respectively,indicating that timing of melt activity in these two areas are different.The anatectic zircons in Ta'erhe leucosomes show much lower ?18O(4.46 ± 0.15‰ vs.1.64 ± 0.41‰)and higher 176Hf/177 Hf ratios(0.281843?0.282113 vs.0.282217?0.282332)than the Triassic metamorphic cores from adjacent melanosome 17NDB17,suggesting involvement of external melts.The leucosomes collected in Ta'erhe are tonalitic,trondhjemitic and granitic in compositions with high K2O/Na2 O ratios(0.18?1.89).In comparison,the Fuanghuangguan leucosomes are tonalitic-trondhjemitic in compositions with relatively low K2O/Na2 O ratios(0.14?0.67).They also show different ?Nd(t)values and initial Sr isotopic compositions(Ta'erhe:-15.7?-8.55,0.7072?0.7090;Fenghuangguan:-3.34?-4.38,0.7054?0.7060),indicating that they originate from different sources.3.Combined with previous studies,we found that the Tongbai and Dabie orogens suffered different subduction-exhumation processes: the subduction-exhumation process of the Tongbai is about 15?25 Myr earlier than that of the Dabie orogen;the onset timing of tectonic extension in the Tongbai orogen might be ca.15 Ma earlier than that of the Dabie orogen.In the post-collisional stage,the formation timings of migmatites are 139?131 Ma and 140?120 Ma for the Tongbai and Dabie orogens,respectively.This difference may be caused by the different evolution of orogen. |
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