Abstract:

The zircon UPb geochronology,major and trace elements and SrNd isotopes determination of the Lamasu intrusion in western Chinese Tianshan were conducted to understand its petrogenesis and geodynamic setting. The Lamasu intrusion is mainly composed of quartz monzodiorite，granodiorite porphyry and tonalite porphyry. Among them the quartz monzodiorite was formed in 394.8±4.9 Ma，while the granodiorite porphyry and tonalite porphyry were intruded in 380.9±3.9 Ma. The rocks display the adakitic affinity and show a calcalkaline and highK calcalkaline differentiation trend. They display REE patterns characterized by relative enrichment of LREE ((La/Yb)N: 3.5515.52) with slightly negative or positive Eu anomalies (δEu：0.531.12). They have high Sr ((322808)×10-6) and low Y contents ((12.9018.86)×10-6) and also enriched in LILE but depleted in HFSE with negative Nb，Ta and Ti anomalies. Initial SrNd isotopic compositions are εNd(t)=-4.29 to 0.75 and ISr=0.706052 to 0.708263. The Nd model ages vary from 1.03 to 1.46 Ga. The Pb isotopic ratios of granodiorite porphyry and tonalite porphyry are 206Pb/204Pb=18.50019.044，207Pb/204Pb=15.57515.626，208Pb/204Pb=38.44338.864，while the quartz monzodiorite are 206Pb/204Pb=18.69418.711，207Pb/204Pb=15.62215.630，208Pb/204Pb=38.64838.660. All these geochemical data demonstrate that the Lamasu intrusion is likely derived from the partial melting of oceanic crust，with the subsequent melts interacted during their uplift with the overlying mantle wedge metasomatized by the subducted sediments and subordinately contaminated by ancient crust. The Lamasu intrusion was emplaced in a continental arc setting during the southward subduction of the Junggar Remnant Ocean Basin in the Late Devonian and are closely related with Cu (Au) mineralization of that region.

Key words: Lamasu intrusion, zircon SHRIMP U-Pb age, element geochemistry, isotope geochemistry, western Tianshan