Geochemistry, zircon U-Pb age and its geological significance of the gneissic granite from the eastern segment of the Tatelekebulake composite granite in the south Altyn Tagh
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摘要:
出露于阿尔金构造带南缘北部塔特勒克布拉克复式花岗质岩体东段的片麻状花岗岩, SiO2为71.88%~73.92%, Al2O3为13.39%~14.14%, K2O+Na2O为8.18%~8.85%, K2O/Na2O=1.54~2.33, A/CNK介于1.02~1.09之间, 属高钾钙碱性系列的弱过铝质岩石。该岩石富集大离子亲石元素(LILE), 亏损Nb、Ta、P、Ti等高场强元素(HFSE); ∑REE较高且变化范围大(∑REE=98.57×10-6~579.1×10-6, 平均338.8×10-6), 具有明显的负Eu异常(δEu=0.22~0.71, 平均0.34), LREE相对富集, 轻重稀土分馏明显。Nb/Ta=11.78~15.83、Zr/Hf=34.94~36.82及Th/U=8.4~12.7, 结合源岩判别图解, 推断其源区物质主要来源于上地壳的变泥砂质沉积岩类。微量元素及稀土元素特征暗示原岩部分熔融残留相的矿物组合可能为石榴石+斜长石+金红石, 全岩Zr饱和温度计计算结果显示部分熔融温度>800℃, 推断该岩石形成于麻粒岩相条件下云母和角闪石脱水诱发的部分熔融。该岩石LA-ICP-MS锆石U-Pb原位定年结果为: 761±54Ma(上交点年龄), 451±1.7Ma(核部)和411.3±1.8Ma(边部), 锆石核部Th/U平均为0.64, 边部Th/U平均为0.05。结合区内超高压榴辉岩的原岩形成时代、峰期变质与退变质时代分别为约750Ma、500Ma与450Ma的研究资料(Liu et al., 2012)综合分析, 塔特勒克布拉克片麻状花岗岩的原岩时代为782.3±6.9Ma, 可能与罗迪尼亚超大陆裂解事件有关; 花岗岩结晶年龄为451±1.7Ma, 形成于俯冲碰撞造山后抬升阶段, 对应于区内深俯冲陆壳的折返时代, 此时超高压变质岩石发生了麻粒岩相的退变质作用, 随后411.3±1.8Ma又受到另一期地质事件的改造。
Abstract:The Tatelekebulake gneissic granite, outcropped in the eastern segment of the Tatelekebulake composite granite in the south Altyn Tagh, have a high-K cal-alkaline and weakly peraluminous composition (SiO2=71.88%~73.92%, Al2O3=13.39%~14.14%, K2O+Na2O=8.18%~8.85%, K2O/Na2O=1.54~2.33, A/CNK=1.02~1.09), and are riched in large ion lithophile elements (LILE) and depleted in high field strength elements (HFSE) with a clear negative Nb, Ta, P, Ti anomaly. REE distribution patterns show negative anomaly of Eu (δEu=0.22~0.71), distinct enrichments in LREE relative to HREE. The element ratios, such as Nb/Ta (11.78~15.83), Zr/Hf (34.94~36.82), Th/U (8.4~12.7), combining discriminating compositions of source rocks, indicate the rock is formed by the partial melting of meta politic-arenaceous sedimentary rocks from lower crust. The residual mineral assemblage (Grt+Pl+Ru) which is implied by the trace and REE elements characteristic and the partially melting temperature >800℃ calculated by whole rock Zr thermometer, indicate that the rock was generated by the dehydration from amphibole and biotite under granulite faces. LA-ICP-MS in situ U-Pb dating obtains the three group ages: 761±54Ma (upper intersection point age), 451±1.7Ma (core) and 411.3±1.8Ma (rim). The average Th/U ratios of core (0.64) and rim (0.05), zircon CL images character and regional geological structural setting, suggest that the source rock age is 782.3±6.9Ma, relating with the event of breakup of Rodinia; the rock formation age is 451±1.7Ma, formed in a post-orogenic environment after collision and corresponding to the retrograde metamorphism of UHPM rocks; then the rock underwent a subsequent geologic event at 411.3±1.8Ma.
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Key words:
- South Altyn Tagh /
- Tatelekebulake /
- Gneissic granite /
- Geochemistry /
- Zircon U-Pb age
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图 1
阿尔金造山带地质构造图(a, 据Liu et al., 2002) 及研究区地质简图(b，据广西壮族自治区地质调查研究院, 2003修编)
Figure 1.
Geological and tectonic map of Altyn Tagh (a, after Liu et al., 2002) and sketch map of Washixia area (b)
图 2
塔特勒克布拉克片麻状花岗岩野外露头及显微结构照片
Figure 2.
the outcrop and microstructure photos of the Tatelekebulake gneissic granite
图 3
塔特勒克布拉克片麻状花岗岩A/CNK-A/NK图解(a, 据Peccerillo and Taylor, 1976)和K2O-SiO2图解(b, 据Rickwood, 1989)
Figure 3.
A/CNK-A/NK (a, after Peccerillo and Taylor, 1976) and K2O-SiO2(b, after Rickwood, 1989) plots of the Tatelekebulake gneissic granite
图 4
塔特勒克布拉克片麻状花岗岩稀土元素球粒陨石标准化配分曲线(a, 标准化值据Taylor and Mcclennan, 1985)及微量元素原始地幔标准化蛛网图(b, 标准化值据 (Sun and McDonough, 1989)
Figure 4.
Chondrite-normalized REE patterns (a, normalization values after Taylor and Mcclennan, 1985) and primitive mantle-normalized trace element patterns (b, normalization values after Sun and McDonough, 1989) for the Tatelekebulake gneissic granite
图 5
塔特勒克布拉克片麻状花岗岩的锆石阴极发光图像
Figure 5.
CL images of zircons in Tatelekebulake gneissic granite
图 6
塔特勒克布拉克片麻状花岗岩LA-ICP-MS U-Pb年龄谐和图
Figure 6.
LA-ICP-MS zircon U-Pb concordia diagram for the Tatelekebulake gneissic granite
图 7
塔特勒克布拉克片麻状花岗岩的源岩判别图解(据(Sylvester, 1998))
Figure 7.
Diagrams for discriminating compositions of source rocks for Tatelekebulake gneissic granite (after Sylvester, 1998)
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