Differential subduction and exhumation of crustal slices in the Sulu HP‐UHP metamorphic terrane: insights from mineral inclusions, trace elements, U‐Pb and Lu‐Hf isotope analyses of zircon in orthogneiss
Differential subduction and exhumation of crustal slices in the Sulu HP‐UHP metamorphic terrane: insights from mineral inclusions, trace elements, U‐Pb and Lu‐Hf isotope analyses of zircon in orthogneiss
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DOI：
10.1111/j.1525-1314.2009.00833.x
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发表时间：
2009-12
影响因子：
3.4
通讯作者：
F. Liu;A. Gerdes;H. Xue
中科院分区：
文献类型：
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作者：
F. Liu;A. Gerdes;H. Xue
根据新的证据，将苏鲁造山带从东南向西北划分为高压（HP）地壳I片和超高压（UHP）地壳II片和超高压（UHP）地壳III片。对不同切片的锆石进行了矿物包裹体、阴极发光图像、U‐Pb SHRIMP定年以及原位微量元素和Lu‐Hf同位素分析。锆石颗粒在岩浆原岩结晶、HP或UHP变质和晚期角闪岩相退变过程中形成了三个不同的区域：(1)振荡带状岩心，具有低压（LP）矿物包裹体和Th/U > 0.38；（ii）高发光地幔（Th/U < 0.10），第1片锆石为Qtz + Grt + Arg + Phe + Ap，第2和第3片锆石均为Coe + Grt + Phe + Kfs + Ap的HP矿物包裹体；（iii）低发光边缘，含LP矿物包裹体，Th/U < 0.08。继承岩心的锆石U - Pb SHRIMP分析表明，所有7个正交体的原岩年龄均为785-770 Ma。II片锆石记录的超高压变质作用和随后的退变年龄（分别为c. 228和c. 215 Ma）明显高于III片锆石（分别为c. 218和c. 202 Ma），而I片锆石记录的超高压变质作用和随后的退变年龄更大（分别为c. 245和c. 231 Ma）。此外，6个副长岩的Ar - Ar黑云母年龄从HP第1片（约232 Ma）到UHP第2片（约215 Ma）和UHP第3片（约203 Ma）逐渐降低，解释为角闪岩相退变。综合数据表明，苏鲁造山带的高压变质作用和超高压变质作用的年龄由东南向西北递减。因此，高压-超高压单元被解释为三个地壳切片，它们经历了不同的俯冲和挖掘历史。在分离和挖掘之前，I层在深度约55-65 km处与大陆岩石圈分离，随后被挖掘出来，而II层的俯冲持续到深度约100-120 km （UHP）。第三片经历了与第二片相似的地球动力学演化，然而，超高压变质作用和随后的挖掘都发生在大约10万年前。UHP II和III片中两种正长岩的岩浆锆石岩心显示出相似的中新元古代结晶年龄，但Hf同位素组成差异较大（εHf（~ 785）分别为- 2.7 ~ +2.2和- 17.3 ~ - 11.1），表明它们在Rodinia分裂期间形成于不同的地壳单元（分别为中元古代和古元古代至太古代）。UHP和逆行锆石域的Th/U和176Lu/177Hf均低于新元古代火成岩核，但176Hf/177Hf(t)高于新元古代火成岩核。超高压构造域与逆行构造域的相似性表明，晚退变对超高压变质体系的化学和同位素组成没有明显的影响。
Based on new evidence the Sulu orogen is divided from south‐east to north‐west into high‐pressure (HP) crustal slice I and ultrahigh‐pressure (UHP) crustal slices II and III. A combined set of mineral inclusions, cathodoluminescence images, U‐Pb SHRIMP dating and in situ trace element and Lu‐Hf isotope analyses was obtained on zircon from orthogneisses of the different slices. Zircon grains typically have three distinct domains that formed during crystallization of the magmatic protolith, HP or UHP metamorphism and late‐amphibolite facies retrogression, respectively: (i) oscillatory zoned cores, with low‐pressure (LP) mineral inclusions and Th/U > 0.38; (ii) high‐luminescent mantles (Th/U < 0.10), with HP mineral inclusions of Qtz + Grt + Arg + Phe + Ap for slice I zircon and Coe + Grt + Phe + Kfs + Ap for both slices II and III zircon; (iii) low‐luminescent rims, with LP mineral inclusions and Th/U < 0.08. Zircon U‐Pb SHRIMP analyses of inherited cores point to protolith ages of 785–770 Ma in all seven orthogneisses. The ages recorded for UHP metamorphism and subsequent retrogression in slice II zircon (c. 228 and c. 215 Ma, respectively) are significantly older than those of slice III zircon (c. 218 and c. 202 Ma, respectively), while slice I zircon recorded even older ages for HP metamorphism and subsequent retrogression (c. 245 and c. 231 Ma, respectively). Moreover, Ar‐Ar biotite ages from six paragneisses, interpreted as dating amphibolite facies retrogression, gradually decrease from HP slice I (c. 232 Ma) to UHP slice II (c. 215 Ma) and UHP slice III (c. 203 Ma). The combined data set suggests decreasing ages for HP or UHP metamorphism and late retrogression in the Sulu orogen from south‐east to north‐west. Thus, the HP‐UHP units are interpreted to represent three crustal slices, which underwent different subduction and exhumation histories. Slice I was detached from the continental lithosphere at ∼55–65 km depth and subsequently exhumed while subduction of the underlying slice II continued to ∼100–120 km depth (UHP) before detachment and exhumation. Slice III experienced a similar geodynamic evolution as slice II, however, both UHP metamorphism and subsequent exhumation took place c. 10 Myr later. Magmatic zircon cores from two types of orthogneiss in UHP slices II and III show similar mid‐Neoproterozoic crystallization ages, but have contrasting Hf isotope compositions (εHf(∼785) = −2.7 to +2.2 and −17.3 to −11.1, respectively), suggesting their formation from distinct crustal units (Mesoproterozoic and Paleoproterozoic to Archean, respectively) during the breakup of Rodinia. The UHP and the retrograde zircon domains are characterized by lower Th/U and 176Lu/177Hf but higher 176Hf/177Hf(t) than the Neoproterozoic igneous cores. The similarity between UHP and retrograde domains indicates that late retrogression did not significantly modify chemical and isotopic composition of the UHP metamorphic system.