四川丹巴穹状变质地体

四川西部丹巴地区最为引人注目的地质构造是穹状变质地体的发育。华北、扬子和羌塘三个板块之间的南北向和东西向双向收缩,引起区内发育了大小不等的十几个穹状变质地体,自北而南有马奈、春牛场、丹巴、公差、格宗等变质穹隆。多数穹隆的核部出露的是前寒武纪的片麻岩和混合岩,例如春牛场侵入体。其中的片麻岩原岩、黑云母和角闪石质片麻岩均属本区最老的岩石。它不整合于志留纪地层之下,年代学研究证明其年代属新元古代(大约865~785Ma)。混合岩从形态上说,多为条带状,偶见角砾状的角闪石质混合岩。但是也有不少穹隆,核部是花岗岩类。岩石化学研究证明,它们大多属于S型花岗岩,仅个别为I型。穹状变质地体的外围变质带可分三类:(1)巴罗带型区域递增变质带,有的显示变质带的倒转;(2)巴肯型变质带;(3)低级区域变质带,多数是中压绿片岩相。巴罗带变质的泥质岩,多数变晶矿物如黑云母、十字石、石榴石均具早期低级变质矿物的定向包裹物,显示明显叠加变质的信息。变质泥质岩的∑REE=(195~274)×10-6,(La/Yb)n=0·811~1·917。稀土配分曲线和微量元素蛛网图具Nb、P、Ti负异常,显示大陆地壳的特征,是陆缘碎屑物质区域变质产物。巴肯带出露于丹巴以北,主要变质泥质岩是夕线石片麻岩类,常见铁铝榴石而少见堇青石,说明原岩富铁贫镁,局部出现锌铁尖晶石。由北侧的巴肯带到巴罗型变质的公差穹隆到南部的格宗穹隆变质带是从高温到低温连续变化的。因之,我们倾向于认为松潘—甘孜造山带的东南缘是一个规模较大的、呈NE向分布的低—中压区域变质带,总体是一条热轴,垂直走向,向东南温度逐步降低。据前人同位素年龄资料:M1巴罗型区域变质发生于约210~205Ma,马奈花岗岩U-Pb锆石年龄为(197±6)Ma。M2巴肯型变质与岩体侵入有关,年龄约为164Ma。M3喜马拉雅期重结晶的黑云母年龄约为30Ma。总之,丹巴变质穹隆的形成是青藏高原东北部地质构造演化中重要的一幕,其主要活动期起于印支晚期最后结束于喜马拉雅期的隆升和挤出。依据低压高温变质带的空间分布,推测本区印支末期存在一NE向的热轴,同时也是S型花岗岩体的出露区。至于木里一带穹隆与丹巴穹隆在变质程度上的差异,应当归因于印支晚期的陆壳增厚过程中,北倾南倒逆冲剪切造成区域热流的不均一性。也说明了青藏高原东北缘在喜山期隆升之前具有复杂的构造变质历史。

The Danba domal metamorphic terrane, Western Sichuan, China

The most prominent tectonic feature in the Danba region of western Sichuan province is the development of a domal metamorphic terrane. North-south and east-west two-sided contraction between the North China, Yangtze and Qiangtang blocks had led to the development of more than ten domal metamorphic terranes of various size. From north to south, these include the Manai, Chunniuchang, Danba, Gongchai and Gezong domes. Pre-Sinian gneissic rocks and migmatites are exposed in the core of these domes. For example, in the core of Chunniuchang dome are biotite-hornblende gneissic rocks considered to be the oldest rocks in this region. They are unconformably overlain by Silurian strata. Geochronological study indicates that the age of the gneiss is Neoproterozoic(ca. 865-785 Ma). The migmatites are mostly stromatic, but occasionally agmatitic. There are many granite-cored domes, including those of Rongxuka, Changzheng, Jiajika and Waduo, scattered in areas north of Yajiang. Petrochemical and geochemical study of some of these granites demonstrate that they are S-type granites, but with a volcanic arc geotectonic setting. According to their associated metamorphic rocks the domal terranes can be subdivided into 3 types: (1) Barrovian prograde, but in some cases inverted; (2) Buchan type; (3) low-grade metamorphic rocks of greenschist facies. Most of the Barrovian metapelitic rocks contain index minerals such as biotite, staurolite, garnet, and kyanite occurring as porphyroblastic minerals with numerous mineral inclusions of earlier age. It is obvious that these index minerals are developed across earlier mineral assemblages. The REE distribution pattern,∑REE=(195-274)×10~ -6 , (La/Yb)_n=0.811-1.917 with a negative Eu anomaly, and trace element spider diagrams for the metapelites with Nb, P and Ti negative anomalies show characteristics of the upper continental crust. The Buchan type metapelites are almandine garnet-bearing, but are rare in cordierite, thus indicating high-Fe and low-Mg characteristics of the protolith. In some of the metapelites franklinite is found. The grade of metamorphism is higher in the northwestern part of the Danba region and is lower towards the southeast, as seen in decreasing metamorphic grade from the Gongchai domal terrane towards the Gezong domal terrane. Therefore, it is reasonable to conclude that in the southeastern part of the Songpan-Garze orogenic belt there was a NE-trending low to intermediate-pressure regional metamorphic belt that was also a thermal axis. Perpendicular to this trend metamorphic temperatures declined. Geochronological data showed that the Barrovian prograde metamorphism (M1) was initiated in 210-205 Ma, with peak metamorphic stage at 204-190 Ma, which is correspondent with the zircon U-Pb dating of the Manai Granite yielding (197±6) Ma. Buchan type metamorphism is dated at about 164 Ma (according to sphene U-Pb dating). A Himalayan tectonothermal event is characterized by the biotite growth (ca. 30 Ma) in some of the dynamo-metamorphic rocks.To sum up, the formation of Danba domal metamorphic terrane is one of the important events in the evolution of the northeast Qinghai-Xizang plateau. The major activation age began in late Indosinian time and finished with the uplift and tectonic extrusion in the Himalayan period. According to the spatial distribution of the high temperature metamorphic belt it is interpreted that in late Indosinian time a thermal axis paralleled the southwestern margin of the Yangtze craton and was also a locus for emplacement of Mesozoic S-type granitic intrusions. As to the difference in the grade of metamorphism between the Muli and Danba regions, it can probably be ascribed to the heterogeneity of the regional geothermal flow during continental crustal thickening and a top-south thrusting event since the late Indosinian.