南秦岭勉略带北光头山花岗岩体群的成因及其构造意义

Origin and dynamic significance of Guangtoushan granitic plutons to the north of Mianlue zone in southern Qinling.

南秦岭勉略带北部花岗岩体从闪长岩到花岗闪长岩和花岗岩变化,反映了钙碱性岩岩石组合特征,矿物组成以长石、石英、黑云母和少量角闪石为主,副矿物有锆石、磷灰石、磁铁矿和榍石,岩石化学上它们相对高K、Sr,Zr／Y比值较高,富集LEE和LILE,贫化HFSE,与后碰撞富钾钙碱性I型花岗岩特征一致。此外,它们明显亏损Nb、Ta,低Y、Yb和有较高的 LaN／YbN和Sr／Y比值,多数岩体发育淬冷岩浆结构的暗色闪长质微粒包体,包体与寄主花岗岩的稀土及微量元素存在明显差异,证明它们是地壳增厚背景下,可能由下部地壳拆沉作用导致的分别来自幔源和下部地壳熔融的二元岩浆混合演化的产物。个别高分异岩体的Fetot／Mg比值高、明显亏损Sr、Ba、Ti、P,呈现了向强分异A型花岗岩过渡的后碰撞富钾过铝偏碱性花岗岩特征。因此,结合西部岩体形成年代早于东部岩体分析,西部形成时代较早偏中基性的含有大量闪长质微粒包体的岩体代表了早期下部地壳拆沉作用的发生,东部形成较晚分异程度高的高钾钙碱性Ⅰ型花岗岩体的出现指示了南、北两大陆块碰撞汇聚后向伸展的转折,而更晚期高度分异的姜家坪富钾花岗岩体的出现则表明秦岭造山带已进入主碰撞结束期的伸展拉张演化阶段,并预示了新的板内演化期的到来。

The granitoid plutons to the north of Mianlue zone in southern Qinling have a cale-alkaline association with a range from diorites through granodiorites to granites, composed mainly of feldspars, quarts, biotites and minor hornblendes with accessory minerals of zircons, apatites, magnetites and sphenes. They have high K, Sr and Zr/Y ratios, as well as LEE and LILE enrichment and poor HFSE, similar to those of post-collisional high-K talc-alkaline I-type granitoids. Moreover, they are characterized by low Y and Yb abundances, high LaN/YbN and Sr/Y ratios and depleted in Nb and Ta. The marie microgranular enclaves, displaying an evidently different rear earth and trace elements than those of host rocks, are quite developed in plutons. All of those give evidence that they were resulted from mixture of two-end magmas derived from mantle and lower crust, which were possibly trigged by lower crustal delamination. High differentiation pluton is high in Fe^tot/Mg and depleted in Sr,Ba,Ti,P, showing characteristics of post-collisional high-K peraluminous subalkalic granitoids transformed to strong differentiation A-type granites in the final stage. Thus, based on the older ages of granitoid plutons outcropped in the west area and younger ages in the east, the west plutons with large amounts of enclaves represented the start of lower crustal delamination, and the higher differentiation talc-alkaline I-type granitoid plutons in the east area indicate a transition from collisional convergence to extension between south and north continental blocks. Where the formation of strong differentiation Jiangjiaping high-K granitic pluton demonstrate that Qinling Orogenic Belt evolved into extension period after main collision and promise a coming of new intraplate period.