藏北羌塘地块新生代火山岩中麻粒岩捕虏体的岩石学和地球化学研究：对青藏高原新生代火山岩成因及下地壳性质的约束

Petrology and geochemistry of the granulite xenoliths from Cenozoic Qiangtang volcanic field: Implication for the nature of the lower crust in the northern Tibetan plateau and the genesis of Cenozoic volcanic rocks.

青藏高原北部羌塘地块的新生代高钾钙碱性火山岩中含有很多下地壳捕虏体,这些捕虏体的主要岩石类型为二辉石麻粒岩和单斜辉石麻粒岩。本文对鸟兰乌拉湖南侧枕头崖地区新生代高钾钙碱性火山岩中的9个麻粒岩样品(6个基性麻粒岩和3个酸性麻粒岩)进行了系统的岩石学,矿物化学,地球化学和Sr-Nd-Pb同位素地球化学研究。其中,紫苏辉石具有高MgO+FeO,低Al_2O_3的特征,单斜辉石具有低Al_2O_3和TiO_2的特征,黑云母具有高TiO_2的特征,这些特征都表明这些矿物为变质成因。矿物温压计算表明二辉石麻粒岩形成的平衡温度为783℃～818℃。单斜辉石麻粒岩形成压力在0.845～0.858GPa之间,来源深度约28km。表明它们可能是来自青藏高原加厚陆壳中部的岩石样品。基性麻粒岩的SiO_2=48.76%～58.61%,Al_2O_3=18.34%～24.50%,Na_2O=3.16%～5.41%,K_2O=1.58%～3.01%,低Mg~#(30～67),富集轻稀土(LREE)和大离子亲石元素(LILE),其具有较高的Rb/Sr(0.09～0.21)和(La/Yb)_N(17.32～49.35),具有较低的Nb/Ta (9.76～14.92),其Eu异常变化于0.19～0.89之间。基性麻粒岩的Sr-Nd-Pb同位素地球化学表现为^(87)Sr/^(86)Sr=0.710812～0.713241,ε_(Sr)=169.13～203.88,^(143)Nd/^(144)Nd=0.512113～0.512397;ε_(Nd)=-4.70～-10.05,^(206)Pb/^(204)Pb=18.7000～-18.9565,^(207)Pb/^(204)Pb=15.7135～-15.7662,^(208)Pb/^(204)Pb=39.1090～39.4733。和基性麻粒岩类似,酸性麻粒岩也表现出富集轻稀土(LREE)和大离子亲石元素(LILE)的特征,它们的^(87)Sr/^(86)Sr=0.712041～0.729088,ε_(Sr)=180.71～430.59,^(143)Nd/^(144)Nd=0.512230～-0.512388;ε_(Nd)=-4.74～-7.96,^(206)Pb/^(204)Pb=18.9250～-19.1717,^(207)Pb/^(204)Pb=15.7662～-15.7720,^(208)Ph/^(204)Pb=39.2109～39.6467。上述地球化学特征表明这些基性麻粒岩的源岩是下地壳岩石,而非地幔岩或玄武质堆晶岩,而酸性麻粒岩的源岩极有可能是准铝质的花岗质岩石。这就表明青藏高原新生代下地壳的地温梯度很高,并含有部分沉积岩,而非典型的辉长质下地壳。而且,详细的研究表明,这种特殊的下地壳可能对青藏高原新生代高钾钙碱性和橄榄粗安质岩浆的起源有重要作用。因此目前所认为的超钾质-橄榄粗安质岩浆源于富集岩石圈地幔在对流减薄作用下发生部分熔融的观点值得重新考虑。

High-K calc-alkaline magmas from Cenozoic Qiangtang volcanic field, northern Tibetan Plateau, have entrained lower crustal xenoliths, which vary from two-pyroxene granulite to clinopyroxene granulite. Nine granulite （six mafic granulite and three felsic granulite） xenoliths have been collected from the south Ulan Ul Lake, Hol Xil area, and their petrological characteristics together with preliminary geochemical data are discussed here. The orthopyroxene have high FeO ＋ MgO and low Al2O3, the clinopyroxene have low Al2O3and TiO2 , the biotite have extremely high TiO2 content, these geochemical features suggest that these minerals are metamorphic genetic. Thermobarmetric results indicate that temperature and pressure of 783 to 818℃ and 0. 845 to 0. 858 GPa that equal to 27. 9 - 28.3km depth respectively for all the granulite xenoliths. Geochemically, the mafic granulite display SiO2 = 48. 76% - 58. 61% , Al2O3 = 18. 34% -24. 50% , Na2O = 3.16% - 5.41% , K2O = 1.58% - 3.01% , low Mg^# （30 -67） , enriched in light rare earth elements （LREE） and large ion lithophilous elements（LILE） with high ratios of Rb/Sr （ （0. 09 - 0. 21 ）, （ La/Yb）N （ 17. 32 - 49. 35 ）, low ratios of Nb/Ta（9. 76 - 14.92） and variable Eu anomalies （δEu =0. 19 -0. 89）, they also have more evolved Sr-Nd-Pb isotopic composition that those of host dacite with ^87Sr/^86 Sr = 0. 710812 and 0. 713241, εSr = 169. 13 - 203. 88, ^143Nd/^144 Nd = 0. 512113 - 0. 512397 ; ^Nd = - 4. 70 - 10. 05, ^206Pb/^204 Pb = 18. 7000 - - 18. 9565, ^207Pb/^204 Pb = 15. 7135 - - 15. 7662, ^208Pb/^204Pb=39. 1090 - 39. 4733. Like the mafic granulite, the felsic granulite show enrichment of LREE and LILE and evolved Sr-Nd-Pb isotopic composition with （La/Yb）N = 2. 04 - 10.82, ^87Sr/^86 Sr = 0. 712041 - 0. 729088, εSr = 180. 71 - 430. 59, ^143Nd/^144Nd =0. 512230 - - 0. 512388;εNd = - 4. 74 - - 7. 96, ^2006Pb/^204 Pb = 18. 9250 - - 19. 1717, ^207Pb/^204 Pb = 15. 7662 - - 15. 7720, ^208Pb/^204Pb = 39, 2109 - 39, 6467. These petrological and geochemical data clearly indicate that the protoliths of the mafic granulite is upper crustal rocks rather than mantle-derived basaltic rocks or basaltic accumulates, and the protoliths of the felsic granulite maybe metaluminous granitic rocks or upper crustal rocks. Then it is clear that the lower crust in northern Tibetan Plateau is a hot and partially metasedimentary instead of wholly gabbroic. Further mineralogy and geochemical study suggest that interaction between mantle derived magma and metasedimentary lower crust have played an important role in the generation of shoshonitic rocks and high-K calc-alkaline andesite-dacite rock association. Then the currently popular model that the Tibetan uhrapotassic-shoshonitic rocks were derived partial melting of enriched lithospheric mantle and the nature of the lithospheric mantle in northern Tibet should be reconsidered.