Spatio-temporal variations of Late Triassic granitic magmatism in the Lincang batholith and implications for post-collisional process along the Paleo-Tethys suture in SW China

The most striking Lincang granitic batholith widely distributed in the Sanjiang Paleo-Tethys Orogen provides us a crucial window to understand the subduction, consumption, and associated geodynamic mechanism of the Paleo-Tethys Ocean during the late Paleozoic to early Mesozoic. In this study, we carry out an integrated study of petrology, zircon U-Pb geochronology, whole-rock major and trace elements, and in situ zircon Hf isotope of the Late Triassic granitic rocks from the Lincang granitic batholith, with the aim of investigating their geochronological framework, spatio-temporal variations, genetic mechanism, and tectonic significance. The granitic rocks presented in this study consist of granodiorite and monzogranite, with minor syenogranite. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U-Pb dating of these granitic rocks yield crystallization ages of 237.6–209.0 Ma, indicating multi-stage granitic magmatic activities during the Late Triassic. Geochemical analyses show that these granitic rocks belong to medium-K calc-alkaline and shoshonite series, and have metaluminous to peraluminous characteristics and low whole-rock zircon saturation temperature (726–834°C). The Late Triassic granitic rocks show transitional characteristics and originate from different magma sources, indicative of transitional I-type and S-type granite affinities. The 237.6 Ma gneissic granodiorites belong to I-type granite and have typical continental arc granites of enrichments in large ion lithophile elements (Rb, K, Pb, and Th) and light rare earth elements (LREEs), and depletion in high field strength elements (Nb, Ta, Ba, Ti, and P) and heavy rare earth elements (HREEs), with parental magma derived from garnet amphibolite facies lower crust. Whereas the 222.7–209.0 Ma granitic rocks have S-type granite affinities with psammitic source. The transitional geochemical characteristics led us to favour that syn-collisional stage in response to final closure of the Paleo-Tethys Ocean in Southwestern Yunnan lasted at least until 237.6 Ma. Subsequent extensional mechanism and associated asthenosphere upwelling after final closure of the Paleo-Tethys Ocean induce large-scale partial melting of the middle-lower crust, resulting in voluminous generation of S-type and A-type granitic rocks and associated volcanic rocks in Southwestern Yunnan.

广泛分布于三江古特提斯造山带（Sanjiang Paleo-Tethys Orogen）的极具代表性的临沧花岗岩基（Lincang granitic batholith），为我们理解晚古生代至早中生代期间古特提斯洋的俯冲、消亡及其相关地球动力学机制提供了关键研究窗口。本研究针对临沧花岗岩基的晚三叠世花岗岩类岩石开展了岩石学（petrology）、锆石U-Pb年代学（zircon U-Pb geochronology）、全岩主量与微量元素（whole-rock major and trace elements）以及原位锆石Hf同位素（in situ zircon Hf isotope）的综合研究，旨在阐明其年代学格架、时空变化规律、成因机制及构造意义。本研究涉及的花岗岩类岩石包括花岗闪长岩（granodiorite）、二长花岗岩（monzogranite），以及少量正长花岗岩（syenogranite）。采用激光剥蚀电感耦合等离子体质谱法（LA-ICP-MS）对这些花岗岩类岩石进行锆石U-Pb定年，结果显示其结晶年龄介于237.6~209.0 Ma之间，表明晚三叠世存在多期花岗质岩浆活动。地球化学分析结果表明，该套花岗岩类岩石属于中钾钙碱性系列与钾玄岩系列，具有准铝质至过铝质特征，且全岩锆石饱和温度较低（726~834℃）。晚三叠世花岗岩类岩石呈现过渡性特征，起源于不同的岩浆源区，兼具I型花岗岩（I-type granite）与S型花岗岩（S-type granite）的亲缘性。形成年龄为237.6 Ma的片麻状花岗闪长岩属于I型花岗岩，具有典型的大陆弧花岗岩特征：富集大离子亲石元素（large ion lithophile elements）（Rb、K、Pb、Th）与轻稀土元素（light rare earth elements，LREEs），亏损高场强元素（high field strength elements）（Nb、Ta、Ba、Ti、P）与重稀土元素（heavy rare earth elements，HREEs），其母岩浆起源于石榴角闪岩相下地壳。而形成年龄为222.7~209.0 Ma的花岗岩类岩石则具有S型花岗岩亲缘性，源区为砂质岩。这种过渡性地球化学特征表明，滇西地区古特提斯洋最终闭合对应的同碰撞阶段至少持续至237.6 Ma。古特提斯洋最终闭合后的伸展机制及伴随的软流圈上涌，引发了中-下地壳的大规模部分熔融，导致滇西地区大规模发育S型与A型花岗岩（A-type granite）及其配套火山岩。