Diverse Partial Melting during Continental Rifting, Subduction-Exhumation and Mountain-Root Collapse in the Dabie Orogen, Central China
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摘要: 大别造山带发育了与大陆俯冲-折返和碰撞造山等相关的不同构造岩石单位.针对存在的问题，本项研究开展了宿松变质带、中大别超高压带和北大别杂岩带等不同俯冲岩片花岗质岩石的野外地质调查以及岩石学、元素-同位素地球化学和锆石年代学等方面系统研究.研究结果表明：（1）宿松变质带花岗片麻岩的原岩时代包括晚太古代（2.5~2.7 Ga）和新元古代（770~830 Ma）两大类，其中新元古代花岗片麻岩的原岩是由经历了~2.0 Ga变质作用的晚太古代岩石在新元古代大陆裂解过程中发生重熔作用形成的；（2）首次揭示中大别花岗片麻岩至少包含两种不同的原岩时代（~750 Ma和780~800 Ma）与岩石成因，并在三叠纪俯冲-折返期间经历了~230 Ma和~220 Ma两期部分熔融作用；（3）北大别混合岩中发育折返早期（209±2 Ma）因高温减压而引起的黑云母脱水熔融以及山根垮塌期间（110~145 Ma）有水加入的加热熔融（水致熔融）形成的多种浅色体；（4）发现并限定了北大别变质闪长岩是在燕山期山根垮塌期间，由三叠纪深俯冲的新元古代镁铁质下地壳岩石发生部分熔融作用而形成的.因此，这为大别造山带在新元古代大陆裂解、印支期地壳的俯冲-折返及燕山期山根垮塌期间发生的多种部分熔融作用提供了新的制约.Abstract: The Dabie orogenic belt consists of a series of fault-bounded lithotectonic units with various metamorphic grades and evolutional histories related to continental subduction and exhumation. Based on the existing controversy and problem in the region, this study performed the detailed field investigation, and petrological, element-isotope geochemical and zircon geochronological researches on the granitic rocks from different subducted slices such as the Susong metamorphic zone (SZ), Central Dabie ultrahigh-pressure metamorphic zone (CDZ) and North Dabie complex zone (NDZ). The results suggest that:(1) the precursor ages of the granitic gneisses in the SZ can be divided into two types, i.e. the Late Archean (2.5-2.7 Ga) and the Neoproterozoic (770-830 Ma), and Neoproterozoic protoliths of them were derived from remelting of the Late Archean rocks with~2.0 Ga metamorphic overprinting during the Neoproterozoic continental rifting; (2) the granitic orthogneisses in the CDZ at least contain two kinds of precursor ages of~750 Ma and 780-800 Ma with different petrogenesis, which underwent two episodes of partial melting at~230 Ma and~220 Ma during the Triassic subduction and exhumation; (3) there are a variety of leucosomes, formed by high-T decompression-induced biotite dehydration melting during the initial stage of exhumation at~209 Ma and water-fluxed heating melting corresponding to the mountain-root collapse at 110-145 Ma, within the migmatites in the NDZ; (4) the meta-diorites from the NDZ were discovered and are documented to be resulted from partial melting of the Triassic deep-subducted mafic lower-crustal rocks during the mountain-root collapse in the Early Cretaceous. Thus, all these provide new constraints on diverse partial melting during the Neoproterozoic continental rifting, the Triassic crustal subduction-exhumation and the Cretaceous mountain-root collapse in the Dabie orogen, Central China.
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图 1 大别山造山带的地质简图
BZ.北淮阳带；NDZ.北大别杂岩带；CDZ.中大别超高压变质带；SDZ.南大别低温榴辉岩带；SZ.宿松变质带；HMZ.浒湾混杂岩带；HZ.红安低温榴辉岩带；DC.角闪岩相大别杂岩；XMF.晓天-磨子潭断裂；WSF.五河-水吼断裂；HMF.花凉亭-弥陀断裂；TSF.太湖-山龙断裂；TLF.郯庐断裂；SMF.商城-麻城断裂; DWF.大悟断裂.据Liu et al.(2007a)修改
Fig. 1. Schematic geological map of the Dabie orogen
图 2 宿松变质带花岗片麻岩(样品11TZL1和1303WJH)的锆石εHf(t)⁃U-Pb年龄
红色符号为宿松变质带花岗片麻岩，黑色符号为中大别超高压带花岗片麻岩；据李远等(2018)
Fig. 2. Zircon εHf(t) values and U-Pb ages as identified by the SHRIMP U-Pb dating for the granitic gneisses (samples 11TZL1 and 1303WJH) from the Susong metamorphic zone
图 3 中大别碧溪岭花岗片麻岩的代表性锆石阴极发光(CL)图像及U-Pb同位素年龄
Fig. 3. Cathodoluminescence (CL) images and U-Pb ages of representative zircons from the Bixiling granitic gneiss in the Central Dabie UHP metamorphic zone
图 4 北大别混合岩中不同类型浅色体的野外照片
a和b.塔儿河，发育多种类型浅色体的混合岩；c.鹿吐石铺，含转熔石榴子石的浅色体；d.鹿吐石铺，转熔石榴子石减压分解为角闪石和斜长石，外围又被石英+长石环绕；据刘贻灿等(2019)
Fig. 4. Field photographs showing various leucosomes within the migmatites from the North Dabie complex zone
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