西藏羌塘中部绒马地区石榴蓝闪片岩变质演化过程的视剖面模拟及其意义

Pseudosection modelling of garnet blueschist from Rongma Area,central Qiangtang,North Tibet: Implications to the tectonic evolution of central Qiangtang.

藏北羌塘地体中部产出一变质杂岩带,因其地貌突起,将羌塘地体一分为二,故常称其为羌中隆起带。虽然在该变质杂岩带中先后识别出蓝片岩、榴辉岩等变质岩,但对该变质杂岩带演化过程的认识却存在截然不同的观点,一种观点认为该变质杂岩带是原位的古特提斯板块缝合带的标志;另一观点则针锋相对,认为该变质杂岩带系外来的、底辟上升的杂岩带,不能作为古板块缝合带的证据。在该变质杂岩带中部的绒马乡,石榴蓝闪片岩呈大小不一的岩片和/或透镜体产出于石榴石多硅白云母石英片岩内,主要由石榴石变斑晶和由蓝闪石、绿泥石、白云母、绿帘石、石英、钠长石、金红石/钛铁矿、磷灰石、黑云母等矿物构成的基质组成。石榴石变斑晶粒径达2 mm,具典型的生长环带：核部富锰,锰铝榴石分子摩尔分数可达22%,至边部,铁铝榴石和镁铝榴石分子含量显著升高,而锰铝榴石分子含量则快速下降。石榴石变斑晶内部包体发育,可再细分为两类,一类包体产出于核部,包括被解释为硬柱石假象的细粒钠云母,绿帘石所构成的细粒板状集合体和细粒富铁蓝闪石、石英及金红石等包体;另一类包体则为数量较少的自形大颗粒绿帘石包体,产出于石榴石边部,在其内部还有细粒蓝闪石、金红石与石英等包体产出。基质中的角闪石可识别出3期：核部为富铁的蓝闪石,幔部为贫铁的蓝闪石,最外部为冻蓝闪石。基质中的绿帘石和绿泥石常为自形,绿帘石内常见细粒蓝闪石、石英、金红石等包体,而绿泥石边部常有黑云母的冠状体。在PEXPLE程序计算的p-T视剖面图中,石榴石核部形成的p-T条件为2.0 GPa、470℃,对应硬柱石榴辉岩相,而石榴石边部形成的p-T条件为1.7-1.8GPa、530-540℃,对应绿帘石榴辉岩相。岩相学观察与p-T视剖面模拟研究充分反映了绒马地区石榴蓝闪片岩分别经历了硬柱石榴辉岩相、绿帘石榴辉岩相和近等温快速降压的退变质等变质过程,系冷洋壳快速俯冲与折返的产物,因此,文中支持该变质杂岩带为原位古特提斯板块缝合带的观点。硬柱石转变为绿帘石时,在俯冲通道中释放了大量的流体。T-O视剖面研究进一步表明这种矿物相转变只发生于高氧逸度条件下,暗示所释放的流体可能也是高氧逸度流体。该高氧逸度流体可交代上覆地幔楔,并诱发后者发生部分熔融作用形成高氧逸度岩浆,如果这一推测是合理的,则羌塘地块内部应该存在斑岩型铜金矿床。

Within Qiangtang basin, northern Tibet, a metamorphic belt divides the Qiangtang basin into the northern and southern blocks and was designated as the central Qiangtang metamorphic belt. Although blueschist and eclogite were distinguished within this belt, the tectonic evolution of this metamorphic belt still remains largely debatable. One interpretation is that the central Qiangtang metamorphic belt marks an iwsitu palaeo-Tethyan suture zone between northern Oiangtang terrane of Cathaysian affinity and southern Qiangtang terrane of Gondwana affinity. The alternative argument suggests that the central Qiangtang metamorphic belt was underthrust southward beneath the Qiangtang terrane along the Jinsha suture zone about 200 km in the north. Therefore, the central Qiangtang metamorphic belt did not mark an irrsitu suture zone within Qiangtang terrane. Here we investigate the garnet-bearing blueschists from Rongma area of central Qiangtang in northern Tibet, where is critical to understanding the tectonic evolution of the central Oiangtang metamorphic belt. The mineral assemblage in these rocks is characterized by porphyroblastic garnet set within a matrix of fine-grained amphibole, white mica, epidote, chlorite, albite and quartz with accessory rutile, titanite and apa- tire. The Mn content of the garnet porphyroblasts decreases from core to rim, whereas the Fe and Mg contents show an increasing trend, respectively. Fine-grained rhomb-shaped inclusions of paragonite and epidote in as- sociation with quartz occurring within the core of the garnet porphyroblasts are interpreted as pseudomorphs after lawsonite. Large grains of epidote with inclusions of Na amphibole, rutile and quartz occur in the matrix in contact with the rims of the garnet grains. In the matrix, we identified two stages of Na amphibole rimmed by Na-Ca amphibole and albite. Retrograde chlorite is rimmed by fine-grained biotite. On the basis of micro- structural observations and pseudosection modelling, we trace the p-T path for the Rongma garnet blueschist from 2. 0 GPa and 470℃ （lawsonite-stability field） to about 1.7 GPa and 530-540℃ （epidote eclogite field）, marking an initial increase in temperature and decrease in pressure. This stage is followed by a decrease of pressure through the blueschist facies down to p-T conditions of about 0. 6 GPa and 530 ℃. In combination with previous work including the available isotopic age data, the p-T path Obtained in the present study sug-gests deep subduction of palaeo-Tethyan oceanic crust between southern and northern Qiangtang blocks, sup- porting the model that the blueschist belt defines the location of the palaeo-Tethyan suture zone within north- ern Tibet. Since the transformation of lawsonite to epidole is known to release water along the suhduction channel, our results also provide clues to a better understanding of the subduction-related Late Permian to early Triassic andesitic magmas in northern Tibet. Furthermore, p-T psendosection has revealed that the epidote was formed during high 0 fugacity, implying that the new formation of fluid was also of high O fugacity. The free fluid with high O fugacity moving upward triggered off the partial melting of mantle wedge to form high oxidation magmas. This implies that gold rich porphyry copper deposits occurred within Qiangtang terrane.