东秦岭磨沟碱性岩体年代学、地球化学及岩石成因

Geochronology,geochemistry and petrogenesis of the Mogou alkalic pluton in the East Qinling orogenic belt

河南嵩县磨沟碱性岩体位于华北陆块南缘,其主体岩性为霓辉正长岩,局部出露正长岩和少量似斑状(霓辉)正长岩,发育岩浆暗色包体。LA-ICP-MS锆石U-Pb定年结果显示,霓辉正长岩和正长岩成岩年龄分别为210.4±2.0Ma(MSWD=0.39)和237.6±2.9Ma(MSWD=0.95),结合前人研究成果,认为该岩体是中三叠世和晚三叠世两期岩浆活动的产物。岩石地球化学特征显示,霓辉正长岩的σ值为8.93~11.08,K2O含量为10.70%~13.76%,K2O/Na2O值为3.87~10.35,属于过碱性岩石系列的钾质碱性岩;在微量元素组成上,富集轻稀土和大离子亲石元素,亏损高场强元素,具有弱或不明显的负铕异常。在同位素组成上,霓辉正长岩具有偏高的、变化范围很大的(87Sr/86Sr)i值和显著偏低的εNd(t)值;锆石的εHf(t)值变化于-21.6^-4.5之间,两阶段Hf模式年龄为1 555~2 629 Ma。磨沟岩体是由含金云母的富集地幔部分熔融产生的幔源岩浆与壳源岩浆混合并经一定程度的分异演化形成的,岩石中高含量的K来源于这种含金云母的富集地幔。磨沟岩体早期岩浆活动发生于秦岭造山带主碰撞峰期的局部伸展环境,晚期岩浆活动则发生于后碰撞伸展环境。

The Mogou alkalic pluton, located in the southern margin of the North China Block, is mainly com- posed of aegirine-augite syenite, with minor syenite and porphyritic （aegirine-augite） syenite as well as marie magma enclaves. LA-ICP-MS U-Pb dating of the aegirine-augite syenite yielded two ages of 237.6±2.9 Ma （MSWD = 0.95） and 210.4±2.0 Ma （MSWD= 0.39）, suggesting the middle and late Triassic epoch, as also shown by previous studies. The aegirine-augite syenite is of peralkaline series and belongs to potassic alkaline rock with σ being 8.93--11.08, K20 10.70% --13.76%, and K20/Na20 3.87--10.35. The REE and trace elements are enriched in LREE and large ion lithophile elements, and depleted in high-field strength elements, with slightly negative to no obvious anomalies of Eu. The aegirine-augite syenite shows high （87Sr/86Sr）i values with a large variation and significantly negative eNd（ t ） values. The zircon Hf isotopic compositions are characterized by εHf（t） values of - 21.6 to -4.5, and the two-stage model ages of 1 555 to 2 629 Ma. The formation of the Mogou pluton could be interpreted as mixing/mingling of crustal-melted magma and mantle-derived magma which originated from phlogopite-bearing enriched mantle by low degree partial melting. The early stage mag-matic activity of the Mogou pluton occurred in a local extension setting during a great regional collision period, while the late stage magmatism took place in the post-collision environment.