Petrogenesis of an early Eocene gabbro–granite complex in Kachang (SW Yunnan) and its implications for Eocene magmatism in the Tengchong terrane of SW China

Major and trace element, zircon U–Pb and Hf-isotope data are reported for mafic intrusions and host granite from the Kachang pluton (North Yingjiang of SW Yunnan) in the Tengchong Terrane, in order to investigate their sources, petrogenesis and tectonic implications. The zircon U–Pb age of the mafic rocks (~55 Ma) is identical to that of the host granite (56.7 ± 0.6 Ma). The mafic rocks have high MgO concentrations (up to 13.43 wt.%) at low SiO₂ contents (low to 42.73 wt.%) and slight negative to positive <i>ε</i>Hf(<i>t</i>) values (−2.26 to +0.59). They are enriched in LILE and LREEs and depleted in HFSEs, which can be explained as melts derived from a enriched mantle, with some crustal contamination. The host granite have high SiO₂ contents (69.18–72.65 wt.%), highly negative <i>ε</i>Hf(<i>t</i>) values (−9.08 to −5.14), suggesting mainly derived from an ancient crustal source. Field observations, geochronology, geochemistry and zircon Hf isotopic compositions point to a complex petrogenesis, where enriched mantle- and crust-derived magma mixing was coupled with crystal fractionation, thus explaining the genetic link between mafic and felsic magmas, result of mafic magma upwelling triggered by the subduction rollback of the Neotethyan slab. Our new data, along with the data reported (especially zircon U–Pb dating and Lu–Hf isotope data) in the Tengchong Terrane, indicate that the spatial and temporal variations and changing magmatic compositions over time in the Tengchong Terrane closely resemble those of the Lhasa Terrane in southern and central Tibet.

本文报道了滇西腾冲地体盈江北部卡昌岩体中镁铁质侵入体及寄主花岗岩的主量元素（major element）、微量元素（trace element），以及锆石U-Pb（zircon U–Pb）和Hf同位素（Hf-isotope）数据，旨在探讨其源区、岩石成因及构造意义。该岩体中的镁铁质岩石锆石U-Pb年龄约为55 Ma，与寄主花岗岩的年龄（56.7±0.6 Ma）一致。镁铁质岩石在低SiO₂含量（低至42.73 wt.%）时具有高MgO含量（最高达13.43 wt.%），且εHf(t)值呈轻微负至正的区间（-2.26至+0.59）。它们富集大离子亲石元素（LILE）与轻稀土元素（LREEs），亏损高场强元素（HFSEs），该特征可被解释为源自富集地幔的部分熔融熔体，并受到了一定程度的地壳混染。寄主花岗岩具有高SiO₂含量（69.18–72.65 wt.%），εHf(t)值为强负值（-9.08至-5.14），表明其主要来源于古老地壳源区。野外地质观察、年代学数据、地球化学特征及锆石Hf同位素组成均指示其具有复杂的岩石成因：富集地幔源与地壳源的岩浆混合作用与结晶分异作用耦合，由此解释了镁铁质与长英质岩浆之间的成因联系，该成因过程由新特提斯洋板片（Neotethyan slab）回撤引发的镁铁质岩浆上涌所驱动。本文的新数据结合腾冲地体已发表的相关数据（尤其是锆石U-Pb定年与Lu-Hf同位素数据）表明，腾冲地体岩浆作用的时空分布与岩浆组成随时间的演化特征，与西藏中南部的拉萨地体（Lhasa Terrane）极为相似。