东昆仑石榴石闪长玢岩的形成机制及其对埃达克岩成因的制约

Petrology and geochemistry of garnet-bearing dioritic prophyry of east Kunlun:Implications for genesis of adakitic rocks

本文报道在东昆仑地区发现的一种较为稀少的含石榴石英云闪长玢岩,这也是此类岩石在中国的首次发现。该次火山岩形成于晚三叠纪,主要由富钙(CaO>5 wt%)、贫锰(MnO<3 wt%)的石榴石,富铝的角闪石(>15.9 wt%),中性斜长石和石英等斑晶以及基质物质组成。岩石含有中等的SiO_2(61.1～62.2wt%),低的MgO(<2.0 wt%),K_2O(<1.3 wt%)以及较高的Al_2O_3(>17 wt%)含量,呈现出次铝质至轻微过铝质的特征(ACNK=0.89～1.05)。在微量元素方面,该岩石富集大离子亲石元素和轻稀土元素,同时亏损Nb-Ta-Ti,显示出典型的消减带特征。而异常低的重稀土(Yb<0.8×10^(-6))和相对高的Sr/Y比值(约38)表明石榴石是一个残留相,而较高的Al_2O_3含量,大多为正的铕异常反映了斜长石因结晶受到抑制而在岩浆演化晚期的聚集,同时表明岩浆具有较高的水含量。Nd-Sr同位素组成(ε_(Nd)(t)=-2.33～-1.38;^(87)Sr/^(86)Sr=0.706 5～0.706 7)和斜长石的反向分带显示,壳幔间岩浆混合作用在岩体的形成过程中扮演了重要角色。石榴石斑晶和其中的钛铁矿包体均含有较低的MgO,且包裹石榴石的角闪石形成于较高的压力(8～10 kb)条件下,显示这些矿物结晶自一个长英质岩浆中,且很可能形成于壳幔过渡带附近。尽管该岩石在富铝、低重稀土、高Sr/Y等很多方面均类似于埃达克岩石,但其中等含量的Sr(<260×10^(-6))和La/Yb比值(16～21)却明显低于典型的埃达克岩和太古代的TTG。结合石榴石斑晶中广泛存在的磷灰石包体以及其较高的Sr和轻稀土分配系数,本文提出在岩浆演化早期结晶的磷灰石有效地降低了残余熔体中的Sr和轻稀土。这进一步表明,即使在高水逸度的条件下,磷灰石在岩浆早期的大量结晶可以有效地阻止一些弧岩浆演化成为埃达克质岩石。

This paper reports a garnet-bearing dioritic porphyry, which was first recognized in east Kunlun, NW China. The rocks formed in the late Triassic and consisted mainly of Ca-rich （ CaO 〉 5 wt% ）, Mn-poor （ MnO 〈 3 wt% ） garnet, Al-rich hornblende （ 〉 15.9 wt% ）, andesitic plagioclase,quartz phenocrysts and matrix. The rock possesses intermediate SiOz （61. 1 - 62.2 wt% ） ,low MgO（〈2.0 wt% ） ,K20（〈1. 3 wt% ） and relatively high A1203（〉17 wt% ）, showing metaluminous to slightly peraluminous characteristics（ ACNK =0.89 -1.05）. On the aspect of trace elements, the rock is enriched in large ion lithosphile elements and light rare element elements and depleted in Nb-Ta-Ti, displaying typical features of subduction-related magma. The extremely low HREE （ Yb 〈0. 8×10^-6） and relatively high Sr/Y ratios （ - 38） indicate garnet was a residual phase,while relatively high A1203 , mostly positive Eu-anomalies suggest accumulation of plagioclase in late stage of magma due to depressed crystallization by high water content. Nd-Sr isotopic composition（εNd（t）=-2.33～-1.38;^87Sr/^86Sr=0.7065～0.7067） and reversed zoning of plagioclase imply an important role of magma mixing during the genesis of the rock. Garnet phenocrysts and ilmenite inclusions enclosed in garnet contain relatively low MgO, and hornblende enclosing garnet formed under relatively high pressure （8 10 kb） , suggesting that these minerals most likely crystallized from a silicic magma probably around the crust-mantle transition zone. Although the rock is very similar to adakites on various aspects, such as Al-rich, HREE-poor and high Sr/Y ratios,its intermediate Sr contents（ 〈260×10^-6） and La/Yb ratios（ 16 -21 ）are strikingly lower than those of typical adakites and Archean TTG. Considering high partitioning parameters of Sr and LREE of apatite and the common occurrence in garnet,we propose that crystallization of apatite may effectively scavenge Sr and LREE from melt. This suggests that,even under high water fugacity, substantial crystallization of apatite during early stage of magmatic evolution is able to prevent some arc magmatism from evolving into adakitic rocks.