中國新疆阿什庫勒盆地新生代火山岩之地球化學與岩石成因

Geochemistry and Petrogenesis of Cenozoic Volcanic Rocks in the Ashikule Basin of Xinjing, China

林瑋幗(Wei-Kuo Lin)

指導教授 ： 宋聖榮

國立臺灣大學/理學院/地質科學研究所/碩士(2013年)

https://doi.org/10.6342/NTU.2013.00564

摘要

阿什庫勒盆地位於青藏高原西北緣的西昆侖中，松潘-甘孜地體之上，坐落在阿爾金斷層與其分支康西瓦斷層之交匯處。阿爾金斷層為一條長約兩千公里的大型走向滑移斷層，橫越青藏高原之西北緣。前人認為阿什庫勒盆地是阿爾金斷層在印度板塊碰撞時，受到向北移動的帕米爾地塊折彎而形成的拉分盆地。在盆地內分布了許多年輕火山，其中最近一次的噴發活動曾被當地報紙記載。
阿什庫勒盆地之新生代火山岩具有高鉀的特徵，其二氧化矽含量變化大，自基性至酸性都有。根據其微量元素與鍶釹同位素之特徵，將之分為基性至中性、中酸性和酸性三組火山岩。基性至中性火山岩之不相容元素分布與其他西昆侖地區新生代火山岩相近，具有明顯Ta-Nb-Ti虧損的特徵，並且具有高鍶同位素比值與低釹同位素值之特徵，顯示其來自於富化的源區。前人認為這些火山岩可能來自於受到前期隱沒作用時交代變質作用而富化的岩石圈地函。而酸性火山岩卻不具有如基性至中性火山岩中Ta-Nb-Ti虧損的特徵，且有較高的鍶同位素比值。這樣的差異無法以結晶分化等作用解釋，因此認為可能是源區本身的性質造成的差異。岩象學觀察中，其含有許多具有如圓蝕外型與峽灣構造等重融構造的石英，指示其源區應富含石英，因此認為阿什庫勒酸性火山岩之源區應為長英質的地殼。中酸性火山岩因具有Ta-Nb-Ti虧損的特徵，所以可能是基性至中性火山岩更進一步結晶分化的結果，且可能受到地殼混染或岩漿混合的作用影響，使之具有介於前述兩者之間的性質。根據前人鉀氬定年和本研究之氬氬定年結果，阿什庫勒火山岩之噴發年代皆為第四紀。除此之外，殼源與函源之火山岩年代上並無明顯差異，應為同時期形成。
歸結出可能造成阿什庫勒之地殼與地函同時發生部分熔融的機制，包含軟流圈上湧和大型走向滑移斷層之剪切生熱等兩種機制。相較於青藏高原中北部，阿什庫勒盆地所在之西昆侖地區其地質環境並不相同。根據前人地球物理資料顯示，此區深部沒有明顯軟流圈上湧的情形，且可能是印度板塊低角度的下插，因此認為軟流圈上湧的機制無法解釋阿什庫勒地區火山的成因。在前人模擬結果中顯示大型走向滑移斷層中，剪切生熱的結果可以造成同時地殼和地函溫度的升高而發生部分熔融。近期研究也顯示隨著溫度升高，岩石散熱的能力會明顯下降，因此由斷層剪切產生的熱能夠累積而達固熔溫度，進而發生部分熔融。由於阿什庫勒盆地為阿爾金斷層之拉分盆地，受到阿爾金斷層的影響甚大，因此本研究認為阿什庫勒盆地中新生代火山岩之成因，可能是受到阿爾金斷層剪切生熱作用的結果。

關鍵字

阿爾金斷層 ； 西崑崙 ； 青藏高原 ； 剪切生熱

並列摘要

Ashikule Basin, a pull-apart basin, is located in the West Kunlun where is on the Sonpan-Ganze terrane in geology and bounds the northwest margin of Tibetan plateau. It sits on the junction of Altyn Tagh Fault and Karakax fault. The Altyn Tagh fault is a large continental strike-slip fault extended about 2000 km and bounds the north margin of Tibetan plateau. There are tens of volumetrically limited volcanoes in the basin. The volcanoes in the Ashikule basin are one of the youngest one on the plateau, the latest eruption was reported by the Xinjiang newspaper at 1951.
The Cenozoic volcanic rocks in the Ashikule basin are potassium rich with wide range of SiO2 contents. According to their concentrations of trace elements and Sr-Nd isotopic characteristics, they are divided into three groups: the basic, intermediate and silicic. The basic group shows Ta-Nb-Ti depletion with high 87Sr/86Sr and lowεNd. These characteristics are similar to other mafic volcanic rocks occurred in the West Kunlun, and were thought to be originated from the metasomatized lithospheric mantle. On the other hand, the silicic group does not show Ta-Nb-Ti depletion, and has higher 87Sr/86Sr value. They are thought to be originated from the felsic upper crust because fractionation from the basic group cannot explain these differences. Meanwhile, they contain abundant quartz with re-melting structure which suggests these silicic rocks are from a quartz-rich source region. The intermediate group is fractionated from the basic group and possibly contaminated by crust or mixed with crustal-origin melts. According to the K-Ar dating from previous research and the Ar-Ar dating in this study, the ages of both crustal- and mantle-origin melts are coeval.
Heating from upwelled asthenosphere and shearing of Altyn Tagh fault are two possible mechanisms to generate both mantle- and crustal-derived melts. Unlike the scenario in northern central Tibet, geophysical data do not suggest that there are hot mantle beneath the Ashikule basin. Meanwhile, the subducted India plate possibly prevents the heating from the asthenosphere. Simulated work on shear heating suggests that it is possible to induce both crustal- and mantle-origin melts by shearing of continental strike-slip fault. Recent researches suggest the conductivity is low in high temperature, so the heat can accumulate and cause partial melting at the long-lived shear zone. Ashikule basin is a pull-apart basin of Altyn Tagh fault and is highly influenced by the fault; therefore, shear heating is the most possible melting mechanism for the Ashikule volcanism in terms of our new data.

並列關鍵字

Altyn Tagh Fault ； West Kunlun ； Tibetan Plateau ； shear heating

參考文獻

Arnaud, N. O., P. Vidal, P. Tapponnier, P. Matte, and W. M. Deng, 1992, The high K2O volcanism of northwestern Tibet: Geochemistry and tectonic implication, Earth and Planetary Science Letters, 111, 351-367.

Brooks, C., Hart, S. R. and Wendt, I., 1972, Realistic use of two-error regression treatments as applied to Rubidium-Strontium data. Reviews of Geophysics and Space Physics, 10, 551-577.

Burg J.P. and Gerya T.V., 2005. The role of viscous heating in Barrovian metamorphism of collisional orogens: thermomechanical models and application to the Lepontine Dome in the Central Alps. J. Metamorphic Geol., 23, 75-95.

Cooper, K. M., Reid, M. R., Dunbar, N. W. and Mcintosh, 2002, Origin of mafic magmas beneath northwestern Tibet: Constraints from 230Th-238U disequilibria. Geochemistry Geophysics Geosystems, v. 3.

Chung, S. L., Chu, M. F., et al., 2005. Tibetan tectonic evolution inferred from spatial and temporal variations in post-collisional magmatism. Earth-Science Reviews, 68, 173–196.