Petrogenesis and zircon U-Pb ages of the Late Paleozoic high-potassic Sonkul and Kokturpak plutons in Kyrgyz Tien Shan, with implications for related skarn-porphyry W-Au(-Cu-Mo) mineralization

The Sonkul and adjacent Kokturpak multiphase plutons are situated along the ‘Major Structural Line of Tien Shan’ (known also as the ‘Nikolaev Line’) that extends for over 500 km and divides the terranes of the Middle and Northern Tien Shan. The plutons comprise a broad variety of high-potassic igneous rocks including the early olivine gabbro, intermediate monzonite to quartz monzonite, and later granodiorite, monzogranite, and leucogranite-alaskite, in turn followed by the late mafic dikes. U-Pb dating of zircon grains yields ages of 299 ± 2 Ma (olivine gabbro), 300 ± 3 Ma to 298 ± 3 Ma (monzonite to quartz monzonite), 299 ± 3 Ma to 298 ± 4 Ma to 297 ± 4 Ma (granodiorite), and 289 ± 4 Ma to 285 ± 2 Ma (monzogranite). These consecutive intrusive phases appear to have been produced by partial re-melting of a partially crystallized (mush) magma batch at deeper levels, which is evident by the presence of zircon antecrysts dated at some 306–311 Ma (up to 323 Ma?). Geochemical signatures of the igneous rocks correspond to high-K calc-alkaline to shoshonitic series intrusions emplaced in a transitional subduction-related to post-collisional setting. A low-degree partial melting of the metasomatically enriched upper mantle can be envisioned as the leading process of shoshonitic magma generation; this was followed by amphibole fractionation in a deep (lower crustal ?) magma chamber and possibly a mantle-induced generation of granitic magmas in the crustal protolith. Further mixing/mingling of the mafic magma and crustal granitic magma was followed by the magma fractionation and emplacement at shallow crustal levels. Assimilation of the old continental crust is consistent with the presence of ancient xenocrystic zircons (dated at ca. 1.5 to 2.5 Ga) in the studied rocks. The Late Carboniferous to Early Permian age and high-K calc-alkaline to shoshonitic composition are similar to those of highly mineralized intrusions of the other segments of the Middle Tien Shan. In this regard, several magmatic pulses of high-potassic intrusions and corresponding metallogenic pulses can be suggested in this region. Whereas the early (ca. 337 to 315 Ma) igneous suites accompanied by dominantly porphyry Cu-Au-Mo mineralization, the later (ca. 305 to 285 Ma) suites including the Sonkul and Kokturpak plutons are accompanied by W- and/or Au-dominant mineralization. This mineralization appears to pre-date more substantial intrusion-related to orogenic-type Au mineralization.

索库勒（Sonkul）与邻区科克图尔帕克（Kokturpak）多期深成岩体沿“天山主要构造带（Major Structural Line of Tien Shan）”展布，该构造带亦被称为“尼古拉耶夫线（Nikolaev Line）”，延伸逾500千米，分隔了中天山与北天山的地体（terranes）。该套深成岩体包含多种高钾火成岩组合，依次为早期橄榄辉长岩（olivine gabbro）、中性二长岩（monzonite）至石英二长岩（quartz monzonite），以及晚期花岗闪长岩（granodiorite）、二长花岗岩（monzogranite）和淡色花岗岩-阿拉斯加型花岗岩（leucogranite-alaskite），后续还发育晚阶段镁铁质岩脉（mafic dikes）。对锆石颗粒（zircon grains）的U-Pb定年（U-Pb dating）结果显示：橄榄辉长岩的形成年龄为299±2 Ma，二长岩至石英二长岩的年龄区间为300±3 Ma至298±3 Ma，花岗闪长岩的年龄区间为299±3 Ma至298±4 Ma再至297±4 Ma，二长花岗岩的年龄区间为289±4 Ma至285±2 Ma。这些连续的侵入期次被认为源于深部环境中部分结晶（糊状）岩浆批次的部分重熔，这一机制可由年龄约306~311 Ma（最高可达323 Ma？）的先晶锆石（zircon antecrysts）得到佐证。火成岩的地球化学特征显示其属于高钾钙碱性至橄榄安粗岩系（shoshonitic series）侵入体，形成于俯冲相关至后碰撞的过渡构造环境。交代富集型上地幔（metasomatically enriched upper mantle）的低度部分熔融被认为是橄榄安粗质岩浆形成的主导过程，后续经历了深部（下地壳？）岩浆房（magma chamber）内的角闪石分异作用，且可能存在地幔诱发的地壳原岩（crustal protolith）花岗质岩浆形成作用。镁铁质岩浆与地壳花岗质岩浆进一步混合/混熔后，经历岩浆分异作用（magma fractionation）并侵位至浅地壳环境。古老大陆地壳的同化作用（assimilation）与研究岩样中产出的年龄约1.5~2.5 Ga的古老捕掳锆石（xenocrystic zircons）特征一致。晚石炭世（Late Carboniferous）至早二叠世（Early Permian）的形成年龄以及高钾钙碱性至橄榄安粗岩系的岩石组合特征，与中天山其他段的高矿化侵入体特征相似。据此可认为该区域存在多期高钾侵入岩岩浆脉冲及相应的成矿脉冲：其中早期（约337~315 Ma）火成岩组合伴生以斑岩型Cu-Au-Mo矿化为主的矿化作用，而晚期（约305~285 Ma）的岩套（包括索库勒与科克图尔帕克深成岩体）则伴生以钨（W）和/或金（Au）为主的矿化作用。该类矿化作用早于与造山型金矿化（orogenic-type Au mineralization）相关的更为大规模的侵入作用。