新疆南天山老虎台地区混合岩地质特征、年代学与锆石Hf-O同位素组成OA

Migmatization,recording the crustal anatexis,plays a vital role in continental crustal evolution;it can occur at various stages of orogenesis and is considered direct petrological evidence for crustal growth and reworking.The South Tianshan Orogenic Belt(STOB),located in the southwestern segment of the giant Central Asian Orogenic Belt,formed through the long-term subduction of the South Tianshan Ocean and subsequent continental collision.This study presents systematic geochronological,whole-rock geochemical,and zircon Hf-O isotope analyses of migmatite from the Laohutai area in the South Tianshan.Our results show that these migmatites originated from the partial melting of supracrustal rocks dominated by volcanic components.Based on their geochemical characteristics,the migmatite samples can be divided into two groups:Leucosome-dominated samples consistently exhibit negative Nb,Ta,and Sr anomalies,with insignificant Eu anomalies;melanosome-rich samples demonstrate not only negative Nb and Ta anomalies but also pronounced positive Sr and Eu anomalies.All samples present a relatively gentle right-inclined rare earth element(REE)pattern,with enrichment in light rare earth elements(LREE)and depletion in heavy rare earth elements(HREE).The U-Pb ages of anatectic zircons from both leucosomes and melanosomes range predominantly from 436 to 376 Ma,indicating regional anatectic events that occurred mainly during the Middle Silurian to Late Devonian periods.The isotopic compositions of anatectic zircons from the melanosomes include εHf(t)values ranging from-15.98 to-2.06(TDM2:3.0～1.6 Ga)and δ18O values ranging between 4.14‰ and 9.93‰.Anatectic zircons from the leucosomes yield comparable values,with εHf(t)ranging from-15.7 to-1.69(T DM2:2.4～1.6 Ga)and δ18 O ranging from 4.30‰ to 10.02‰.In combination with previous studies,this study establishes the genetic link between the southward subduction of the South Tianshan Ocean and the anatectic events.A regional tectonic transition occurred at ca.420 Ma.From～460 to 420 Ma,low-angle,advancing subduction created a compressional regime in the STOB and the northern Tarim margin.Under these conditions,supracrustal rocks were underthrusted into deep crustal levels,generating limited anatexis.Following the tectonic transition,retreating subduction(or slab rollback)dominated from～420 to～390 Ma.This phase was accompanied by extensive mantle underplating and a significant rise in the geothermal gradient,placing the upper plate in an extensional setting.Intense anatexis occurred in response to this tectonic transition.