Subduction Initiation and Generation of the Xigaze Ophiolite, Southern Tibet

<正>The Cretaceous Xigaze ophiolite is best exposed at the central Yarlung Zangbo Suture Zone(YZSZ,Tibet)which also includes the Gangdese arc and the Xigaze forearc basin.This study reports new geochronological and geochemical data for this ophiolite to revisit its geodynamic and petrogenetic evolution.The Xigaze     

藏南罗布莎蛇绿岩辉绿岩中锆石SHRIMP测年

罗布莎蛇绿岩是雅鲁藏布江蛇绿岩带东段出露较好,也是研究程度较高的蛇绿岩片之一。对该蛇绿岩的形成时代至今存在分歧。本文获得该蛇绿岩辉绿岩中的锆石SHRIMPU-Pb定年的结果为162.9±2.8Ma,表明该蛇绿岩的形成时代为中侏罗世,同时发现,在罗布莎地区新特提斯洋形成时,可能存在地幔熔融的岩浆在扩张脊上涌过程中俘获老洋壳或者深海沉积物中锆石的情况;并且在侵位过程中,在该蛇绿岩的局部由于蚀变而产生了变质锆石。     

In-situ Moissanite in Dunite: Deep Mantle Origin of Mantle Peridotite in Luobusa Ophiolite, Tibet

We report the discovery of an in-situ natural moissanite as an inclusion in the Cr-spinel from the dunite envelope of a chromitite deposit in Luobusa ophiolite, Tibet. The moissanite occurs as a twin crystal interpenetrated by two quadrilateral signal crystals with sizes of 17 μm× 10 μm and 20 μm× 7 μm, respectively. The moissanite is green with parallel extinction. The absorption peaks in its Raman spectra are at 967-971 cm-1, 787-788 cm-1, and 766 cm-1. The absorption peaks in the infrared spectra are at 696 cm-1, 767 cm-1, 1450 cm-1, and 1551 cm-1, which are distinctly different from the peaks for synthetic silicon carbide. Moissanites have been documented to form in ultra-high pressure, high temperature, and extremely low fO2 environments and their 13C-depleted compositions indicate a lower mantle origin. Combined with previous studies about other ultra-high pressure and highly reduced minerals in Luobusa ophiolite, the in-situ natural moissanite we found indicates a deep mantle origin of some materials in the mantle sequence of Luobusa ophiolite. Further, we proposed a transformation model to explain the transfer process of UHP materials from the deep mantle to ophiolite sequence and then to the supra-subduction zone environment. Interactions between the crown of the mantle plume and mid-ocean ridge are suggested to be the dominant mechanism.     

西藏罗布莎康金拉铬铁矿区地幔橄榄岩锆石SHRIMP U-Pb年龄及地质意义

本文对罗布莎蛇绿岩康金拉11号铬铁矿矿体的近矿围岩纯橄岩和方辉橄榄岩分别进行了锆石SHRIMP U-Pb年代学研究。结果显示两个样品的年龄都非常发散:纯橄岩的时代跨越早白垩世到太古宙,年龄125.6±2.6~1791±23Ma,4粒锆石的加权平均年龄为130.0±2.8Ma(2.1%)。而方辉橄榄岩的年龄208.9±3.7~2770±44Ma,从晚三叠世到太古宙,2粒锆石的加权平均值为209.7±5.2Ma(2.5%),锆石的复杂年代学结果反映该岩石经历的多阶段地质演化过程。激光拉曼分析显示纯橄岩和方辉橄榄岩中的锆石含磷灰石、长石和石英为主的地壳物质包裹体,暗示其地壳成因,记录了地幔经历了地壳混染过程。这些物质存在于由洋底扩张形成的蛇绿岩中,表明俯冲下去的大陆地壳物质在地幔循环中被运移到海底扩张脊并出露地表。结合我们在康金拉地幔橄榄岩和铬铁矿中发现的大量异常地幔矿物,可能显示了一个地壳物质俯冲至地幔,而后从洋脊回到地表的物质循环过程,佐证了豆荚状铬铁矿多阶段多期次的演化模式。     

西藏日喀则蛇绿岩构造环境再讨论

对日喀则地区群让、白朗和白岗的地幔橄榄岩和辉长辉绿岩进行了主微量元素和铂族元素(PGE)分析,结果显示,三地的地幔橄榄岩PGE总量相似,球粒陨石标准化曲线大致平坦,IPGE和PPGE的分馏不显著,白岗和群让样品的PdN/IrN和PdN/PtN值大于1;白岗村蛇绿岩糜棱岩带中采集到洋岛型碱性玄武质辉绿岩,其化学性质与仁布洋岛型玄武质岩石相似。表明日喀则蛇绿岩起源于弧后盆地环境,其南部的蛇绿岩糜棱岩带可能包含了日喀则蛇绿岩形成时相对应的岛弧火山岩,以及新特提斯洋的残留物。     

西藏雅鲁藏布江缝合带中段日喀则地幔橄榄岩中发现金刚石等异常矿物

本文报道了雅鲁藏布江缝合带中段日喀则蛇绿岩地幔橄榄岩中发现与该带东西段蛇绿岩岩体中相似的金刚石和特殊地幔矿物群。日喀则地幔橄榄岩体以方辉橄榄岩为主,含少量二辉橄榄岩和纯橄岩,辉石岩和辉长岩呈脉状产在方辉橄榄岩中。通过重砂分选实验,在465kg的方辉橄榄岩大样中发现了30余种特殊矿物群,包括金刚石、自然金、自然铬、自然铜等自然元素矿物类;碳硅石等碳化物类;方铁矿、赤铁矿、磁铁矿、刚玉、铬尖晶石、金红石、锡石、黑钨矿等氧化物类;镍黄铁矿、方铅矿、辉钼矿、辉锑矿、闪锌矿、毒砂等硫化物类;Ag-Au等合金矿物类;橄榄石、辉石、锆石等硅酸盐岩类;白钨矿等钨酸盐岩类;萤石等氟化物类。金刚石和自然金、自然铬、自然铜连同碳硅石的发现表明岩体存在强还原环境,这套特殊矿物组合指示日喀则地幔橄榄岩可能与雅鲁藏布缝合带其他蛇绿岩一样,经历了深地幔演化过程。     

Evolution History of Mantle Peridotites in the Xigaze Ophiolite: Constraints from Whole-rock and Mineral Geochemistry

正Ophiolites along the E-W trending Yarlung-Tsangpo Suture(YTS),which separates the Indian plate from the Eurasian plate,have been regarded as relics of the NeoTethyan Ocean.The Xigaze ophiolite in the central YTS     

Detachment‐Accretion Formed the Xigaze Ophiolite along the Southern Margin of the Eurasian Plate in Southern Tibet

Mantle peridotites predominate over the 130–120 Ma mafic rocks, which have a limited temporal interval. This characteristic distinguishes the ophiolite outcrops along the Yarlung Zangbo Sutrue Zone (YZSZ) in southern Tibet from more typical ophiolites, such as the Oman ophiolite. Here we present new structural field observations and identify the low‐angle detachment faults of the Xigaze ophiolite. We demonstrate that the extension associated with the detachment system was accommodated by accretion of mafic magma rather than by thinning and dismemberment. U‐Pb zircon ages from three groups of mafic intrusives closely related to the detachment faults show that the detachment system was initiated at ～126 Ma and ceased at ～123 Ma. A detailed study of the entire Xigaze ophiolite assemblage reveals that the ophiolite formed in situ at the southern margin of the Eurasian Plate, instead of being formed from fragments of tectonic slices.     

藏南白朗地区宗卓组混杂岩的物质来源和形成时代——来自岩石学和碎屑锆石U-Pb定年的证据

本文通过对藏南白朗地区宗卓组混杂岩"基质"进行岩石学、碎屑锆石形态学和U-Pb同位素测年等研究,结果表明:"基质"中碎屑岩岩屑成分以中酸性熔岩为主,石英颗粒在阴极发光下主要呈不发光或弱发光,为低温变质成因的自生石英再沉积;碎屑锆石以岩浆成因锆石为主,少量变质成因锆石,多因经历了一定距离的搬运磨蚀作用而呈浑圆状、椭圆状;碎屑锆石年龄绝大多数分布于422~2867 Ma(43.5%)和71~205 Ma(52.2%),在中元古代(1032~1569 Ma)和早白垩世(102~142 Ma)形成两个年龄主峰。综合研究将研究区宗卓组混杂岩中"基质"的形成时代限定为晚白垩统马斯特里赫特期,雅鲁藏布江缝合带北侧冈底斯弧与南侧印度大陆北缘均对其物源具有一定的贡献。     

西藏羌塘南缘早白垩世普让岩体的锆石U-Pb年龄和地球化学特征及其地质意义

普让岩体位于西藏羌塘南缘多龙矿集区西北部,岩体的岩石类型组合为石英二长岩—二长岩—二长闪长岩,侵位于三叠纪日干配错群灰岩中,并形成矽卡岩型磁铁矿。石英二长岩LA-ICP-MS的锆石~(206)Pb/~(238)U加权平均年龄为116±2Ma(MSWD=6.5),与该带上的多龙含矿斑岩成岩年龄基本一致。普让岩体不含白云母,且标准矿物中富含透辉石,具有较高的Al_2O_3,较低的Ti O_2,富集大离子亲石元素(LILE),亏损高场强元素(HFSE)和重稀土元素(HREE),显示Nb和Ta的负异常,具有明显的岛弧型岩石的地球化学特征,属于准铝质I型花岗岩。与多龙含矿斑岩相比,普让岩体具有相似的ε_(Nd)(t)值(-7.22~-6.81),n(~(87)Sr)/n(~(86)Sr)i值(0.707525~0.708243)和古老的Nd的模式年龄(1.29~1.63Ga)。研究表明:普让岩体可能是在班公湖—怒江洋壳北向俯冲的背景下,下地壳重熔,且在上升的过程中经历了分离结晶和地壳混染作用下形成的。多龙外围的岩浆岩带不仅具有寻找斑岩铜矿的潜力,也是形成和寻找的矽卡岩型多金属矿的有利地区。     

西藏中南部花岗岩类岩石中锆石标型特征的研究

西藏各类花岗岩分布极为广泛。锆石在不同时代和不同岩性花岗岩中的标型特征也互有差异。锆石矿物含量从北向南有逐渐降低的趋势;锆石中的Zr/Hf比值是随着岩石的酸度增加和岩体时代变新而降低;锆石的伸长系数是随着岩石中Si-K+Na的含量增加而加大;在冈底斯带花岗岩中的Si和K+Na含量低时,锆石多出现(100)和(111)晶面,而拉轨岗日岩带和喜马拉雅岩带浅色花岗岩中的Si、K、Na含量较高时,锆石则多出现(110)、(311)和(131)晶面;锆石中稀土元素是以重稀土元素为主,其含量占总稀土量的80%以上。     

REE and PGE Geochemical Constraints on the Formation of Dunites in the Luobusa Ophiolite, Southern Tibet

The Luobusa ophiolite, Southern Tibet, lies in the Indus–YarlungZangbo suture zone that separates Eurasia to the north fromthe Indian continent to the south. The ophiolite contains awell-preserved mantle sequence consisting of harzburgite, clinopyroxene(cpx)-bearing harzburgite and dunite. The harzburgite containsabundant pods of chromitite, most of which have dunite envelopes,and the cpx-bearing harzburgites host numerous dunite dykes.Dunite also exists as a massive unit similar to those of themantle–crust transition zones in other ophiolites. Allof the dunites in the ophiolite have a similar mineralogy, comprisingmainly olivine with minor orthopyroxene and chromite and tracesof clinopyroxene. They also display similar chemical compositions,including U-shaped chondrite-normalized REE patterns. Mantle-normalizedPGE patterns show variable negative Pt anomalies. Detailed analysisof a chromite-bearing dunite dyke, which grades into the hostcpx-bearing harzburgite, indicates that LREE and Ir decrease,whereas HREE, Pd and Pt increase away from the dunite. Thesefeatures are consistent with formation of the dunite dykes byinteraction of MORB peridotites with boninitic melts from whichthe chromitites were formed. Because the transition-zone dunitesare mineralogically and chemically identical to those formedby such melt–rock reaction, we infer that they are ofsimilar origin. The Luobusa ultramafic rocks originally formedas MORB-source upper mantle, which was subsequently trappedas part of a mantle wedge above a subduction zone. Hydrous meltsgenerated under the influence of the subducted slab at depthmigrated upward and reacted with the cpx-bearing harzburgitesto form the dunite dykes. The modified melts ponded in smallpockets higher in the section, where they produced podiformchromitites with dunite envelopes. At the top of the mantlesection, pervasive reaction between melts and harzburgite producedthe transition-zone dunites. KEY WORDS: melt–rock interaction; REE; PGE; hydrous melt; mantle; ophiolite; Tibet     

西藏蓬湖西蛇绿岩地球化学及构造背景研究

西藏蓬湖西蛇绿岩隶属于班公湖—怒江缝合带的中段,是藏北湖区较好的堆晶岩剖面之一。形成该堆晶岩时矿物分离结晶顺序为铬铁矿→橄榄石→单斜辉石→斜长石,分离结晶作用具有由富镁质、铁质向富铝质、钙质演化趋势。堆晶岩主量元素组成与世界典型蛇绿堆晶岩成分相似;各种堆晶岩石的微量元素和稀土元素配分曲线总体上基本一致,反映其同源性;岩石Nb和Ta亏损,稀土元素含量很低,分配模式呈LREE微弱亏损的平坦型。该蛇绿岩形成于弧后盆地扩张中心,属SSZ型蛇绿岩。     

Sensitive High Resolution Ion Micro-Probe U-Pb Zircon Geochronology and Geochemistry of Mafic Rocks from the Pulan-Xiangquanhe Ophiolite, Tibet: Constraints on the Evolution of the Neo-tethys

The Pulan-Xiangquanhe ophiolite in the western Yarlung Tsangpo suture zone of Tibet is investigated for its geochemistry,geochronology,and tectonic implications in detail.Sensitive high resolution ion micro-probe zircon U-Pb dating reveals that diabases in the ophiolite from the three locations of Xugugab,Mapam Yum Co and La'nga Co are dated at 122.3±2.5 Ma,118.8±1.8 Ma and 120.5±1.9 Ma,respectively.These early Cretaceous mafic rocks have Na_2O+K_2O,rare earth element patterns,trace elemental spider diag...     

Sensitive High Resolution Ion Micro-Probe U-Pb Zircon Geochronology and Geochemistry of Mafic Rocks from the Pulan-Xiangquanhe Ophiolite, Tibet: Constraints on the Evolution of the Neo-tethys

The Pulan-Xiangquanhe ophiolite in the western Yarlung Tsangpo suture zone of Tibet is investigated for its geochemistry, geochronology, and tectonic implications in detail. Sensitive high resolution ion micro-probe zircon U-Pb dating reveals that diabases in the ophiolite from the three locations of Xugugab, Mapam Yum Co and La’nga Co are dated at 122.3±2.5 Ma, 118.8±1.8 Ma and 120.5±1.9 Ma, respectively. These early Cretaceous mafic rocks have Na2O+K2O, rare earth element patterns, trace elemental spider diagram and other geochemical fingerprints of typical mature back-arc basin affinity. Therefore, the Pulan-Xiangquanhe ophiolite formed under a mature back-arc basin environment, which was a product of this intra-oceanic subduction system. A suprasubduction system could have existed in the southern margin of Eurasia, which involved both intra-oceanic and continent-ward subductions. Extension dominated the southern margin of the Eurasian continent during the early Cretaceous.     

西藏马攸木金矿松托嘎岩体锆石SHRIMP年龄及其地质意义

马攸木岩金矿床是在西藏普兰县境内发现的大型独立岩金矿床。对矿区西北部规模较大的松托嘎英云闪长斑岩体进行SHRIMP锆石U-Pb同位素年龄测定,所得岩浆结晶锆石的年龄为18.4±1.3Ma(1σ),表明该岩体形成于中新世早期。年轻锆石所包裹的残留锆石核给出两组206Pb/238U年龄值分别为555±25Ma(1σ)和1894±83～1335±56Ma(1σ)。根据野外地质证据,并结合上述同位素年龄数据可以推测北喜马拉雅地区西段在早寒武世经历了一次重要的岩浆热事件,相比之下,松托嘎岩体可能起源于下地壳中—新元古界变质基底的深熔作用。松托嘎岩体成岩时代与雅鲁藏布江东段含矿斑岩成岩年龄(18～12Ma)的一致性表明两者可能受统一的深部地球动力学背景控制。     

Tectonic Evolution of the Dongbo Ophiolite in Western Yarlung Zangbo Suture Zone,Xizang(Tibet)

正The Dongbo ophiolite in the western part of the Yarlung-Zangbo suture zone in southern Tibet rests tectonically on the middle-late Triassic and Cretaceous flysch units,and consist mainly of peridotites,mafic dikes,     

西藏南部江孜地区白垩系的厘定

文中对藏南江孜地区深海一次深海白垩纪地层层序、系统及厚度进行了厘定。建组剖面甲不热、维美等构造复杂,地层层序变化大,重复多。厘定后的岩石层单元有加不拉组、床得组和宗卓组。加不拉组是一套以黑色页岩为主的地层,跨不白垩统一上白垩统三冬阶,下分黑层段和白层段;床得组由紫红色页片状泥灰岩组成,各坎潘期早－中期;宗且即著名的“北家混杂堆积岩”,页岩中含大量外来岩块,时代为坎潘期晚期和马斯特里赫特期。白垩系厚     

Seamount volcanism associated with the Xigaze ophiolite, Southern Tibet

Basaltic lavas at Renbu, Southern Tibet are associated with the Xigaze ophiolite in the Yarlung-Zangbo suture zone. They are alkaline lavas rich in large ion lithophile elements (LILE, Ba, Rb and Sr) and high field strength elements (HFSE, Nb, Ta, Zr and Hf), but poor in Cr, Co and Ni. All of the rocks have chondrite-normalized REE patterns enriched in light rare earth elements (LREE), comparable to modern basalts of the Society Islands, Kerguelen Plateau and Broken Ridge. Abundances of some immobile or moderately immobile elements (Nb, Ta, Zr, Hf, Y, Ti and REE) are also comparable to Kerguelen alkaline basalts. The Renbu basalts are geochemically similar to oceanic island basalts (OIB) and have some elemental ratios, such as Nb/Ta ratios = 15.7–18.1, Th/Nb =  0.06–0.10, La/Nb = 0.59–0.83 and Th/Ta = 1.03–1.52, similar to the primitive mantle. Their ⁸⁷Sr/⁸⁶Sr ratios (0.70453–0.70602) are relatively high, similar to OIB. In the ⁸⁷Sr/⁸⁶Sr vs. εNd(t) diagram, the Renbu basalts plot along a trend from N-MORB to EMII (enriched mantle II), suggesting the involvement of at least two mantle sources in their generation. The Renbu basalts represent seamount volcanism associated with the Xigaze ophiolite. They formed from an OIB-type mantle source within the Neo-Tethyan Ocean that had a composition similar to the modern Indian Ocean mantle.