Multiple Episodes Of Partial Melting Events In The Sulu Orogen And Their Tectonic Implications

Partial melting is a crucial geological process during crustal subduction,exhumation and orogenic collapse,forming melts and fluids which strongly affect the rheological properties of the crust,trigger the exhumation of deeply subducted crust,and promote the element migration and crustal differentiation.Thus,study on partial melting could give great help on understanding the complicated evolution history of orogenic crust.However,the evidence of partial melting is usually erased by later overprinting of deformation,metamorphism and magmatism.In addition,there are significant differences in time,P–T conditions,scale,mechanism of partial melting and melt compositions in the different structural positions of the same orogen.Thus,deciphering the evidence,timing,mechanism,source and melt composition of multiple episodes of partial melting events in orogenic crust are still in need of careful and comprehensive studies on different cases.As one of the largest and well-exposed ultrahigh-pressure(UHP)metamorphic terrane in the world,Dabie–Sulu belt has become a natural laboratory for studying the melt and fluid activities in the deeply subducted crust.It provides precious chance to study the partial melting because of the occurrence of various and multiple-staged partially melted products.A combined study of high precision structure–petrology mapping,petrography,microstructure,zircon geochronology and whole-rock major,trace and Sr–Nd isotope element analyses were carried out to study the partially melted products in Taohang and Haiyangsuo,Sulu orogen.Based on the studies on the in-situ leucosome pockets in the Taohang UHP eclogite,the partially melted timing and melt compositions were constrained,further proposing the new mechanism of the partial melting.In the Haiyangsuo complex,the timing and mechanism of multiple episodes of partial melting events were determined,which constrain the controversial tectonic affinity and evolution of the Haiyangsuo complex.Recent studies have addressed issues such as generation,evolution and crystallization in the UHP eclogites.However,it remains challenging to determine the composition of initial melts in nature due to overprinting processes including melt migration,partial crystallization,melt drainage and retrograde metamorphism.Omphacite-breakdown melting of ultrahigh-pressure eclogite from Taohang is suggested by various lines of evidence:(1)The occurrence of plagioclase as veinlets along grain boundaries surrounding phengite,quartz and zoisite,and has low dihedral angles at triple junctions.(2)The presence of in-situ leucosome pockets composed of plagioclase,euhedral amphibole,minor K-feldspar and epidote,surrounded by coarse-grained garnet,zoisite and omphacite.Fine-grained omphacite relicts scattered throughout the leucosome pockets have significantly lower jadeite content(25–45 mol%)compared with that of the coarse-grained omphacite in the host eclogite(39–56 mol%).Major oxide compositions of 47 leucosome pockets determined based on modes and mineral compositions show they have moderate Si O2(57.48–65.71 wt%),high Na2O(6.73–9.48 wt%),Al2O3(19.31–22.50 wt%)and relatively low Fe2O3T(0.07–4.09 wt%),Mg O(0–5.31 wt%),Ca O(2.11–5.22 wt%),K2O(0.12–1.33 wt%),and Ti O2(0.01–0.20 wt%)contents.In addition,both leucosome pockets and omphacite nearby exhibit enrichment of large ion lithophile elements,U,Pb,Zr,Hf and Ti but depletion of Th and Ta in primitive mantle-normalized spidergram.Both the high Na2 O contents and trace element patterns indicate breakdown of omphacite dominated the partial melting reaction.P–T path of the eclogite constrained by geothermobarometers indicates partial melting took place at P =1.2–1.6 GPa and T = 690–780 °C,followed by crystallization of the melts at the wet solidus.SS-LASS analysis of thin overgrowth rims on zircon from 2 eclogites with leucosome pocket yields weighted mean ages of 215 Ma with Ti-in-zircon temperatures of 567–639 ?(consistent with crystallization temperatures of the leucosome pockets),which implies crystallization of the melt occurring around 215 Ma.In the Sulu belt,the tectonic affinity of the Haiyangsuo complex is controversial due to its polyphase metamorphic history.In this study,most zircon from gneisses and leucosomes has oscillatory-zoned cores with CL-bright overgrowth rims,but some has CL-dark cores or mantles between cores and rims.Oscillatory-zoned cores yield protolith ages of 2.86–2.68 Ga,whereas the mantles and rims yield younger metamorphic/melt crystallization ages of ca.1817–1812 Ma.The mantles are characterized by flat HREE distribution patterns and relatively low Th/U ratios,indicating crystallization during granulite-facies metamorphism.Whereas rims show steep HREE distribution patterns and higher Th/U ratios,suggesting they crystallized from melt.The mantles and rims have ?Hf(t)of-18.2 to-11.0.Using 176Lu/177 Hf = 0.001,these data project back to the array of ?Hf(t)values for the zircon cores.This demonstrates that the thin leucosomes were derived from the gneiss without any mass input from a mantle source.These features are consistent with an origin of the Haiyangsuo as part of the eastern margin of the North China Craton prior to juxtaposition with the Sulu belt.Magmatic zircon from metabasites yields ages of ca.825 Ma,similar to those of scattered metabasite occurrences in the North China Craton;Paleoproterozoic zircon cores(ca.1866–1726 Ma)were scavenged during magma ascent.By contrast,Zircons from the leucogranite dikes show core–mantle–rim structure.Inherited cores yield concordant 206Pb/238 U ages of 776–701 Ma consistent with the dominant protolith ages of the UHP metamorphic rocks in the Sulu belt.whereas overgrowth mantles and rims yield weighted mean ages of ca.220 and 162 Ma,respectively.Both mantles and rims host multiphase solid inclusions and show steep HREE patterns with positive Ce and negative Eu anomalies,representing former melt,suggesting anatexis and crystallization of zircon first during initial decompression and then during orogenic collapse of the Sulu belt;whole-rock Nd and Sr isotope compositions implicate the Sulu belt gneisses as the source of these melts.Our interpretation of these data is that the Haiyangsuo complex has an early geologic history similar to the Jiaobei terrane from the southeastern part of the North China Craton,and was incorporated into the Sulu belt during Triassic collision of the Yangtze and North China Cratons.The two stages of melting relate to Upper Triassic early exhumation,and Upper Jurassic late-stage orogenic collapse,during which the leucogranite magma was derived from a source similar to that elsewhere in the Sulu belt,i.e.,the subducted Yangtze Craton,and not the North China Craton.This shows that during continental collisions,crust from the upper plate may be dragged into the subduction channel,deformed,and subsequently exhumed in association with partial melting of the crust.Combining the previous studies with our research on the multiple partial melting events in the Sulu belt,the evolution model of the Sulu belt is proposed,as follows:(1)During the collisional stage(~240 Ma),south east margin of the North China Craton was dragged into the subduction channel and underwent strong deformation;the deeply-subducted Yangtze crust was locally water saturated leading into hydration melting of a little rock;(2)During the exhumation stage(~220 Ma),the granitic gneisses widely underwent partial melting,and formed migmatite and a few magmatic intrusions.Partially melted eclogites are also recognized in the Sulu terrane;their melting mechanism include dehydration melting of the hydrous minerals(such as phengite,paragonite and zoisite),phase separation of supercritical fluid and omphacite-breakdown melting.The south east margin of the North China Craton dragged into the subduction channel exhumated and overlaid on the Sulu belt,but it was mostly denudated during the following geological history and only a few relicts are preserved in the Sulu belt;(3)During the post-collisional stage(~160 Ma),affected by the subduction of the Pacific plate beneath the Eastern Asian,the Sulu orogen began to be activated.Upper Jurassic magmatism occurring in the northern part of the Sulu belt and in the adjacent Jiaobei terrane records an early stage in the orogenic collapse.