Origin of the Xigaze ophiolite, Yarlung Zangbo suture zone, southern Tibet

The Xigaze ophiolite, (Tibet) displays unusual lithological, petrological, textural, and structural characteristics. There are no large masses of cumulate gabbros, but dolerite intrusives throughout the whole ophiolite sequence, some of which were intruded into already serpentinized peridotites, only minor residual harzburgites and dunites in dominantly Iherzolitic peridotites equilibrated at low temperatures and pressures, and relatively low-temperature deformation structures in the uppermost peridotites. These features suggest a very low heat flow at the spreading center where the Xigaze ophiolite was formed, in good agreement with a discontinuous and slowly-accreting spreading center origin. However, this ophiolite does not represent a typical mid-oceanic ridge ophiolite; rather it was formed in a small basin located at the southern margin of Eurasia, hence within a preexisting oceanic lithosphere. The opening of the nearly N-S Xigaze paleo-ridge resulted from the W-E drift of Africa relative to Eurasia from 180 to 110 Ma. The N-S emplacement onto the continent of the Xigaze ophiolite, formed 120 to 110 Ma ago, can be correlated to changes in direction of motion of the African and Indian plates: a primary intra-oceanic thrusting event probably occurring at 110 or 85 Ma and the final obduction near 50 Ma during the India-Eurasia collision.     

西藏雅鲁藏布构造带日喀则蛇绿岩中新发现豆荚状铬铁矿化

<正>1研究目的(Objective)西藏南部的雅鲁藏布构造带发育一系列中生代蛇绿岩,这些蛇绿岩体东西展布超过2500 km,可分为西段、中段和东段。大多数蛇绿岩体出露大规模地幔橄榄岩而镁铁质岩石出露较少,且地幔橄榄岩中赋存不同规模的豆荚状铬铁矿化,如东段的罗布莎蛇绿岩体赋存大型铬铁矿床,西段的普兰和东波岩体出露小规模铬铁矿化。然而,中段日喀则蛇绿岩一直较少有铬铁矿床(化)报道。笔者近期对日喀则蛇绿岩大竹曲岩体开展了野外考察,并在地     

微量元素和Pb同位素对西藏雅鲁藏布构造带日喀则蛇绿岩形成环境的制约

西藏南部雅鲁藏布构造带分布有一系列蛇绿岩体。人们对这些蛇绿岩体的形成环境仍然存在较大的争议。雅鲁藏布构造带中段日喀则蛇绿岩路曲和大竹曲岩体镁铁质岩石的微量元素和Pb同位素特征指示其母岩浆起源于亏损地幔源区。这些镁铁质岩石的La/Sm和Sm/Yb比值显示其岩浆产生于尖晶石二辉橄榄岩地幔经过大约10%部分熔融作用。综合岩相学和全岩主量元素特征暗示这些镁铁质岩石形成于无水玄武质岩浆。而且这些镁铁质岩石的微量元素和REE元素配分模式均非常相似于N-MORB,除了弱Nb-Ta负异常。这些特征表明路曲和大竹曲岩体形成于大洋中脊环境。此外,路曲和大竹曲镁铁质岩石的Pb同位素结果指示其地幔源区与印度洋MORB地幔域具有相似的地球化学特征。这些镁铁质岩石N-MORB标准化微量元素模式显示弱Nb-Ta负异常可能是由于其地幔源区交代了古老的俯冲带物质。     

Trace and Minor Elements in Smithsonite from Huoshaoyun Nonsulfide Zinc Deposits in Southwest China: A LA–ICP–MS Study

The geodynamic setting of the Xigaze ophiolite has long been debated. Structural and geochemical evidence suggest the Xigaze ophiolite was formed at a slow‐spreading ridge (Nicolas et al., 1981; Liu et al., 2016). Based on incompatible element concentrations, the Xigaze ophiolite volcanics are consistent with the ubiquitous subduction signature in suprasubduction zone (Bedard et al., 2009; Hebert et al., 2012; Dai et al., 2013). It is noteworthy that the Xigaze ophiolite is different from the Geotimes and Lasail and Velly units from Oman ophiolite, respectively. The mafic rocks of the Xigaze ophiolite generally resemble typical N‐MORB and Geotimes volcanics in composition except for slight depletions of Th and Nb (Fig.1a). Although the Xigaze rocks have similar Th and Nb concentrations to Lasail and Velly rocks, most incompatible elements in the Xigaze rocks are comparable to N‐MORB. Petrography in gabbro of Xigaze ophiolite shows that euhedral plagioclases are enclosed by clinopyroxenes suggesting that these minerals have crystallized from an anhydrous magma (Sisson and Grove, 1993). Although the Xigaze volcanic rocks are slightly depleted in Th and Nb, they have MORB‐like trace element characteristics implying that they are derived from an anhydrous MORB magma at spreading centre. Godard et al. (2006) suggested that the mantle source of the Oman ophiolite have element and isotopic characteristics similar to Indian Ocean MORB, where the mantle preserved some older slab materials. A negative Nb anomaly of Oman Geotimes volcanic rocks may be resulted from contamination of the slab materials via decompression melting of the convecting mantle. Moreover, the Xigaze rocks have 1.27–3.18 of (Th/Nb)_N ratios similar with those of Geotimes volcanics ((Th/Nb)_N =0.51–2.77) and lower than those of Lasail and Velly units ((Th/Nb)_N =2.12–6.35). These features suggest that the Xigaze ophiolite may have formed at the spreading centre.     

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.     

Petrology of the mafic rocks of the Xigaze ophiolite,Tibet

The Xigaze ophiolite (Yarlung-Zangbo suture zone, Southern Tibet, China) shows an unusual crustal sequence characterized by a lack of large masses of cumulate gabbros, by dolerites intrusive throughout the whole ophiolite sequence, and by the injection of dolerites in already serpentinized peridotites. The abyssal tholeiitic nature of all the mafic rocks indicates that they have been generated at an oceanic ridge. All the geological arguments and petrological and textural data on the mafic rocks point to very low heat production and large heat losses through widespread intensive sea-water circulation, for the spreading centre in which they have been formed, in good agreement with a slow-spreading ridge origin.     

Burial and exhumation history of the Xigaze forearc basin,Yarlung suture zone. Tibet

The Cretaceous-Eocene Xigaze forearc basin is a crucial data archive for understanding the tectonic history of the Asian continental margin prior to and following collision with India during the early Cenozoic Era. This study reports apatite and zircon(U-Th)/He thermochronologic data from fourteen samples from Albian-Ypresian Xigaze forearc strata to determine the degree and timing of heating(burial) and subsequent cooling(exhumation) of two localities along the Yarlung suture zone(YSZ) near the towns of Saga and Lazi. Thirty-seven individual zircon He ages range from 31.5 ± 0.8 Ma to6.06 ± 0.18 Ma,with the majority of grains yielding ages between 30 Ma and 10 Ma. Twenty apatite He ages range from 12.7 ± 0.5 Ma to 3.9 ± 0.3 Ma,with the majority of grains yielding ages between 9 Ma and 4 Ma. These ages suggest that the Xigaze forearc basin was heated to 140-200 ℃ prior to cooling in Oligocene-Miocene time. Thermal modeling supports this interpretation and shows that the samples were buried to maximum temperatures of ～140-200 0 C by 35-21 Ma, immediately followed by the onset of exhumation. The zircon He and apatite He dataset and thermal modeling results indicate rapid exhumation from ～21 Ma to 15 Ma, and at ～4 Ma. The 21-15 Ma thermochronometric signal appears to be regionally extensive, affecting all the lithotectonic units of the YSZ, and coincides with movement along the north-vergent Great Counter Thrust system. Thrusting, coupled with enhanced erosion possibly related to the paleo-Yarlung River, likely drove Early Miocene cooling of the Xigaze forearc basin.In contrast, the younger phase of rapid exhumation at ～4 Ma was likely driven by enhanced rock uplift in the footwall of north-striking rifts that cross-cut the YSZ.     

西藏日喀则蛇绿岩中石榴角闪岩的矿物、岩石地球化学特征

西藏雅鲁藏布蛇绿岩带中段日喀则地区的白朗蛇绿混杂岩中,存在作为变质底板的石榴角闪岩。本文对其进行了矿物学、变质温压条件和岩石地球化学研究。根据岩相学和矿物化学研究,日喀则石榴角闪岩经历了四个变质阶段,矿物组合分别为:Am_1+Pl_1+Ep_1+Ttn(M1);Grt-c+Cpx-e+Ep_2+Pl_2+Rt(M2);Grt-r+Ep_3/Czo_3+Cpx-l+Am_3+Pl_3+Ttn(M3)和Prh+Ab+Czo+Chl+Cal(M4)。根据SAFMCNHO体系矿物组合演化的视剖面图,结合传统的地质温压计,估算出峰期前的温压条件为560~620℃/9.1~9.8kbar;峰值为830~870℃/18.0~22.0kbar,退变质阶段保留了640~680℃/10.7~14.9kbar的温压条件,最终止于葡萄石-绿纤石的前绿片岩相变质条件;得出一条逆时针的P-T轨迹。白朗变质底板的岩性可分为三类,即石榴角闪岩、角闪辉石岩和单辉角闪岩。三种岩性具有相似的地球化学性质:主量元素具有低钛(0.92%~1.29%)、低钾(0.26%)、低钠(0.24%~2.46%)的特征。稀土元素和微量元素特征与N-MROB相似,富集大离子亲石元素(Rb、Ba、U),亏损部分高场强元素(Nb、Ti、Zr、Hf),表现出白朗蛇绿混杂岩中的石榴角闪岩等与洋中脊玄武岩有亲缘性,且叠加有俯冲上板片SSZ环境特征。白朗石榴角闪岩的存在表明日喀则蛇绿岩在形成后不久就发生了构造就位,体现了大洋内部俯冲/侵位事件。     

南秦岭勉略结合带琵琶寺洋壳蛇绿岩的厘定及其大地构造意义

研究结果表明,康县－琵琶寺－南坪构造带是一个复杂的、含有不同成因岩块的混杂带。该带中分布有蛇绿岩块（古洋壳残片）,为典型的洋脊型拉斑玄武岩,具有轻稀土亏损的稀土配分型式、不活动痕量元素及其比值Th/Yb,Ta/Yb,Ti/Y,Ti/Zr等,指示其来源于亏损的软流圈地幔。琵琶寺蛇绿岩与勉县－略阳地区庄科蛇绿岩以及德尔尼蛇绿岩完全可以类比。因此,康县－琵琶寺－南坪蛇绿构造混杂带乃是勉略带向西延伸的组成部分。     

Platinum-group elemental geochemistry of mafic and ultramafic rocks from the Xigaze ophiolite, southern Tibet

The Xigaze ophiolite in the central part of the Yarlung–Zangbo suture zone, southern Tibet, has a well-preserved sequence of sheeted dykes, basalts, cumulates and mantle peridotites at Jiding and Luqu. Both the basalts and diabases at Jiding have similar compositions with SiO₂ ranging from 45.9 to 53.5 wt%, MgO from 3.1 to 6.8 wt% and TiO₂ from 0.87 to 1.21 wt%. Their Mg^#s [100Mg/(Mg + Fe)] range from 40 to 60, indicating crystallization from relatively evolved magmas. They have LREE-depleted, chondrite-normalized REE diagrams, suggesting a depleted mantle source. These basaltic rocks have slightly negative Nb- and Ti-anomalies, suggesting that the Xigaze ophiolite represents a fragment of mature MORB lithosphere modified in a suprasubduction zone environment. The mantle peridotites at Luqu are high depleted with low CaO (0.3–1.2 wt%) and Al₂O₃ (0.04–0.42 wt%). They display V-shaped, chondrite-normalized REE patterns with (La/Gd)_N ratios ranging from 3.17 to 64.6 and (Gd/Yb)_N from 0.02 to 0.20, features reflecting secondary metasomatism by melts derived from the underlying subducted slab. Thus, the geochemistry of both the basaltic rocks and mantle peridotites suggests that the Xigaze ophiolite formed in a suprasubduction zone.Both the diabases and basalts have Pd/Ir ratios ranging from 7 to 77, similar to MORB. However, they have very low PGE abundances, closely approximating the predicted concentration in a silicate melt that has fully equilibrated with a fractionated immiscible sulfide melt, indicating that the rocks originated from magmas that were S-saturated before eruption. Moderate degrees of partial melting and early precipitation of PGE alloys explain their high Pd/Ir ratios and negative Pt-anomalies. The mantle peridotites contain variable amounts of Pd (5.99–13.5 ppb) and Pt (7.92–20.5 ppb), and have a relatively narrow range of Ir (3.47–5.01 ppb). In the mantle-normalized Ni, PGE, Au and Cu diagram, they are relatively rich in Pd and depleted in Cu. There is a positive correlation between CaO and Pd. The Pd enrichment is possibly due to secondary enrichment by metasomatism. Al₂O₃ and Hf do not correlate with Ir, but show positive variations with Pt, Pd and Au, indicating that some noble metals can be enriched by metasomatic fluids or melts carrying a little Al and Hf. We propose a model in which the low PGE contents and high Pd/Ir ratios of the basaltic rocks reflect precipitation of sulfides and moderate degrees of partial melting. The high Pd mantle peridotites of Xigaze ophiolites were formed by secondary metasomatism by a boninitic melt above a subduction zone.     

西藏班公湖—怒江缝合带东段丁青蛇绿岩中的铬铁矿:产出特征与类型

丁青蛇绿岩体位于班公湖—怒江缝合带东段,该缝合带与雅鲁藏布江缝合带并列,是寻找我国铬铁矿床的重要地区。该蛇绿岩体呈近南东向展布,总面积近600 km2,主要由地幔橄榄岩、辉石岩、辉长岩、辉绿岩、玄武岩、斜长花岗岩、硅质岩和泥质岩组成。根据空间分布,丁青蛇绿岩分为东、西两个岩体。在前人工作基础上,通过地质填图、实测剖面、探槽和钻孔编录,共发现豆荚状铬铁矿矿点83处,其中东岩体27处,西岩体56处。根据铬铁矿产出和围岩特征,丁青铬铁矿可分为4种产出类型。类型I:矿体呈脉状产出,围岩为条带状或透镜状纯橄榄岩和块状方辉橄榄岩;类型II:矿体呈透镜状、豆荚状或不规则团块状产出,围岩为薄壳状纯橄榄岩和斑杂状或块状方辉橄榄岩;类型III:矿体呈浸染状弥散分布于纯橄榄岩中,围岩为条带状纯橄榄岩和块状或斑杂状方辉橄榄岩;类型IV:矿体呈条带状产出,围岩为条带状或透镜状纯橄榄岩和具定向结构的方辉橄榄岩。根据矿石构造特征,主要分为块状、脉状、浸染状、浸染条带状4种类型。块状和脉状铬铁矿为矿石的主要类型,少量为浸染状和浸染条带状,局部纯橄榄岩中发育极少量瘤状或豆状构造。本研究选择了13处代表性铬铁矿点开展了详细的岩石学、矿相学、矿物学和矿物化学等工作。根据矿石中铬尖晶石的矿物化学特征,可将丁青铬铁矿矿体分为高铬(Cr#=78～86)、中高铬(Cr#=60～74)、中铬(Cr#2=30～51)和低铬(Cr#=9～14)4种类型(Cr#=100×Cr/(Cr+Al))。丁青东岩体赋存有中高铬型和中铬型铬铁矿,缺少高铬型铬铁矿;西岩体赋存有高铬型和中铬型铬铁矿,缺少中高铬型铬铁矿。同时在丁青东、西岩体内均发现存在一种Cr#极低的铬铁矿,暂定为"低铬型铬铁矿"。这些不同类型的铬铁矿体与野外产出有一定的对应关系,也可能后者制约了它们的成因。与罗布莎岩体中的典型高铬型铬铁矿对比,丁青豆荚状铬铁矿在矿物组合和矿物化学成分等方面具有许多相似性,认为存在较大的找矿空间。     

西藏雅鲁藏布江缝合带中段仁布豆荚状铬铁矿成因及构造意义

藏南雅鲁藏布江缝合带为目前国内铬铁矿储量最大的缝合带.本文报道了缝合带中段仁布蛇绿岩的豆荚状铬铁矿床,围绕矿床特征开展成因探讨,对缝合带的形成演化和成矿作用提供新制约.仁布蛇绿岩呈近东西走向带状产出,主要由近30个大小不等的地幔橄榄岩体组成.地幔橄榄岩体主要为经历不同程度蛇纹石化的方辉橄榄岩和少量纯橄岩.在纯橄岩和方辉...     

沿雅鲁藏布江缝合带东坡、日喀则等蛇绿岩体磁异常特征及其找矿标志

航空磁测△T图上清楚显示出沿雅鲁藏布江缝合带具有南北两条航磁异常,地质研究确认沿雅鲁藏布江缝合带的蛇绿岩体呈带状东西向延续近2000 km,但已知主要含铬蛇绿岩体分布于雅鲁藏布江南岸。在罗布莎和泽当地表观测到断续分布着含铬蛇绿岩体,地质研究程度较高,岩体规模不大。向西地表有出露日喀则岩体,再西段有当穷、普兰、东坡等规模巨大的岩体。航空磁测图上显示出日喀则、东坡、普兰、当穷等岩体与清晰的航磁异常对应。对上述岩体进行的地面磁测准确地确定了已知岩体与航磁异常的关系,位于雅鲁藏布江东段的罗布莎与泽当岩体属于其南部强航磁异常北缘的负异常范围内的小岩体。西部沿江含铬蛇绿岩体是断续分布的,呈短条带状或较大的三度体;地面磁测异常突出反映出地表岩体磁铁矿分布的不均匀性,在岩体边缘(包括近岩体围岩)的蛇纹石化较强,磁异常亦较强,并由此较准确地圈定出岩体范围。航磁异常反映出岩体内部有相对富积的磁性矿物和磁性体,它们可能来自深部含Fe O、Fe2O3等矿物在岩浆上涌到上地壳由于蛇纹石化而生成了Fe3O4,当温度低于480℃时形成较强的磁性。反演计算了磁性体位于3~5 km深度上,虽然蛇绿岩体及其围岩中存在的磁铁矿非直接来源于地幔物质,但含铬蛇绿岩体的岩浆物质是生成磁铁矿的主要物质贡献者。由此将航磁和地面磁测异常与大地电磁法、反射地震等方法结合可以给出岩体深部结构的地球物理特征,这有利于圈定找矿的远景地段。     

西藏西南部达巴—休古嘎布绿岩带铬铁矿的找矿前景

雅鲁藏布江蛇绿岩带长1500km以上,分为东段(曲水—墨脱)、中段(昂仁—仁布)和西段(萨嘎以西至中印边境)3部分。西段又进一步分为北亚带(达机翁—萨嘎蛇绿岩带)和南亚带(达巴—休古嘎布蛇绿岩带),以往的研究程度很低。达巴—休古嘎布蛇绿岩带的蛇绿岩体(块)由地幔橄榄岩组成,主要岩石类型是方辉橄榄岩和纯橄榄岩,极少量二辉橄榄岩,缺少典型蛇绿岩剖面中的洋壳单元。带内岩体规模大,岩相分带明显,出现较大面积的纯橄榄岩相。拉昂错、东坡、当穷岩体和休古嘎布岩体群发现有较多的铬铁矿化点,矿体呈透镜状(豆荚状)、短脉状、不规则状,矿石以致密块状为主,少部分为浸染状,Cr2O3含量一般达40%以上。矿化类型属于豆荚状铬铁矿。对比研究显示,该岩带的岩体在构造环境,蛇绿岩的类型,岩体规模,岩石组成,保存部位,部分熔融程度,岩石、矿物地球化学特征,铬铁矿化特征等许多方面与国内外大型豆荚状铬铁矿床的含矿岩体相似,成矿条件好,找矿前景较大。提出区内找矿前景最好的地段是拉昂错岩体西北部的纯橄榄岩相,其次是东坡岩体的纯橄榄岩相、纯橄榄岩-方辉橄榄岩相,以及当穷岩体的方辉橄榄岩-纯橄榄岩相。     

西藏永珠蛇绿岩带地质特征

详细地描述了永珠蛇绿岩带的地质特征、岩石化学特征、稀土元素特征和微量元素特征,并把它与典型的蛇绿岩带进行对比,发现它除了具有典型的蛇绿岩带特征外,还具有其特殊性。结合区域沉积环境确定该蛇绿岩带的构造环境为扩张历史暂短、洋陆转换不彻底、发育于消减带上的弧间裂谷盆地。     

西藏日喀则白马让蛇绿岩:亚洲大陆边缘的小洋盆

藏南雅鲁藏布蛇绿岩被认为是新特提斯大洋岩石圈的残留。该带中段的日喀则白马让蛇绿岩是保存较完整的蛇绿岩岩块之一。该蛇绿岩主要由橄榄岩、蛇纹岩、镁铁质侵入岩和玄武岩组成,缺堆晶岩系。镁铁质侵入岩主要呈辉绿岩脉、岩床和少量的岩墙产出。辉绿岩脉在整个蛇绿岩层序中均有分布,侵入橄榄岩的部分岩脉已经变为变辉绿岩和异剥钙榴岩。辉绿岩床(墙)向上逐渐过渡为玄武岩。局部可见日喀则群整合覆盖在玄武岩之上。地球化学分析显示不同产状的镁铁质岩均属于低钾或中钾的拉斑玄武岩,亏损Nb、Ta、Ti和LREE,具有弧前玄武岩(FAB)或弧后盆地玄武岩(BABB)的特征,它们的Ti/V和Yb/V的比值与BABB或正常大洋中脊玄武岩(N-MORB)相似,Sr-Nd-Pb同位素数据指示了亏损地幔(DM)与富集地幔(EM)过渡的源区。镁铁质岩野外产出关系和地球化学特征表明,白马让蛇绿岩的镁铁质岩组合可能形成于SSZ环境。考虑到超镁铁质岩、镁铁质岩和日喀则群在空间上的连续性,认为白马让蛇绿岩可能是起源于亚洲大陆边缘俯冲带上的洋盆,属于原地系统,而非外来的构造岩片。     

西藏达巴—休古嘎布绿岩带铂族元素的分布特征

区内各岩体的地幔橄榄岩中PGE含量及分布型式有所差异.相对于原始地幔,拉昂错和东坡岩体的PGE含量没有显著异常,标准化分布曲线大致平坦,PPGE与IPGE之间分馏不明显,总体特征与中国日喀则地区以及阿尔卑斯和东地中海地区的地幔橄榄岩相似.而当穷岩体的PPGE含量变化范围较大,出现IPGE与PPGE强分馏.单斜辉石岩脉的PGE含量及分布型式与地幔橄榄岩不同:Os、Ir、Ru、Rh亏损而Pt、Pd富集,模式曲线向左倾斜,可能反映其较高程度部分熔融作用和熔体分离作用的产物.铬铁矿石的PGE含量较高,而且变化幅度大(19.7×10-9～5526×10-9),各样品∑PGE平均值(729.33×10-9)是原始地幔的25.6倍.与原始地幔相比,Os、Ir、Ru、Rh总体上表现为较强富集,Pt、Pd富集与亏损并存.其标准化模式曲线较分散,曲线向右倾斜,IPGE与PPGE分馏较强,多数样品呈现Pt负异常.这些特点与中国罗布莎、希腊、南斯拉夫以及塞浦路斯等世界各地蛇绿岩中的铬铁矿床类似.铬铁矿石的PGE含量与相应样品的BMS含量无相关性,PGE不是主要赋存于BMs内.推测PGE主要以PGM的形式存在,具体矿物名称有待进一步发现与研究.     

藏南日喀则弧前盆地帕达那组沉积相分析

藏南桑桑地区帕达那组是上白垩统日喀则弧前盆地浅水沉积。基于野外实测剖面对帕达那组进行了详细的沉积相研究。帕达那组岩相类型主要有五类:砾岩相、砂岩相、粉砂岩相、页岩相和灰岩相。砂岩中发育大量楔状和板状交错层理、平行层理、粒序层理及泥砾、潜穴、生物扰动等沉积构造,页岩中发育潜穴及大量钙质结核。根据岩相和沉积构造特征识别出陆棚相和三角洲相。三角洲相包括前三角洲、三角洲前缘及三角洲平原三个亚相,可进一步划分为水下分支河道、河口砂坝、支流间湾、分流河道、洪泛平原、沼泽等微相。帕达那组地层自下向上发育陆棚相和三角洲相（依次为前三角洲、三角洲前缘及三角洲平原亚相）,整体显示出古水深变浅的趋势。这一沉积环境的变化代表着日喀则弧前盆地由欠充填到过充填阶段的演化。     

日喀则蛇绿岩研究中的几个问题

日喀则蛇绿岩是中国最著名的蛇绿岩,但是,许多问题存在争论,许多现象没有搞清楚。文中讨论了日喀则蛇绿岩中一些重要的现象以及对目前发现的许多矛盾如何考虑的问题。例如,(1)日喀则蛇绿岩中是否存在席状岩墙群?是否存在席状岩床群?本文认为,席状岩墙群应当是存在的,因为有不对称冷凝边出现。而席状岩床群可能是有问题的,岩床作为侵入体可以出现在许多地方,但是,蛇绿岩中不可能出现席状岩床群,尤其还把它作为蛇绿岩岩石组合的一个单元。笔者认为,所谓的席状岩床群可能是席状岩流群,它不是一个独立的单元,是玄武岩单元下部的成员。(2)日喀则蛇绿岩是否统统是蛇绿岩是一个需要考虑的问题,雅鲁藏布江东西两段出现的岩石组合及其产出背景与日喀则地区的蛇绿岩明显不同,其中有些可能未必是蛇绿岩。(3)自1972年彭罗斯会议以来,蛇绿岩研究发展到现在,可能已经进入了一个关键时期。文中回顾了蛇绿岩的定义及其构造意义,指出1972年彭罗斯会议关于蛇绿岩的定义仍然是适用的。蛇绿岩可以概括为洋壳+地幔,这个洋壳来自板块扩张脊,是年轻的;这个地幔是大洋岩石圈地幔,也可能保留有古老岩石圈地幔的印记。蛇绿岩产于板块扩张脊,这是蛇绿岩构造含义的唯一解,蛇绿岩不存在多解性。(4)日喀则蛇绿岩的许多基本观点主要是法国学者提出来的,如关于岩床群的见解,慢速扩张的见解等。但是,上述见解是否都是对的是需要思考的。日喀则蛇绿岩具有得天独厚的条件,我们应当很好利用这个条件,努力把我们的研究做好,为全球蛇绿岩研究贡献我们的一份力量。     

Variably evolved gabbroic intrusions within the Xigaze ophiolite (Tibet): new insights into the origin of ophiolite diversity

Ophiolites worldwide show striking diversities in their rock assemblage and structure (i.e., ophiolite diversity), raising a question whether ophiolites are originally similar before intense tectonic dismemberment. Comparison between ophiolites and oceanic lithospheres at modern mid-ocean ridges may provide key constraints on the origin of ophiolite diversity, because oceanic lithospheric structures are inherently controlled by spreading rates. Here, we present a case study of the Xigaze ophiolite in southern Tibet focusing on its gabbroic intrusions outcropping in three localities, i.e., Dazhuqu, Baigang and Jiding. Compared to the Jiding sequence, the Dazhuqu and Baigang gabbroic rocks are less evolved, characterized by higher Cr₂O₃ contents but lower contents of TiO₂ and rare earth element in both clinopyroxene and bulk compositions. It is evident, hence, that the Xigaze ophiolite is characterized by variably evolved and discontinuously distributed gabbroic intrusions, rather by a continuous lower oceanic crust between the mantle and sheeted dike complex as the Penrose-type ophiolites. Our study, along with the identification in previous studies of oceanic detachment faults within the Xigaze ophiolite, demonstrates that the Xigaze ophiolite shows close similarities to oceanic lithospheres at modern slow- and ultraslow-spreading ridges. Hence, the significant structural distinctions between the Xigaze ophiolite and the Penrose-type ophiolites (e.g., the Oman ophiolite) may be inherently associated with different spreading rates of paleo-ridges. Considering the limited scale of the Xigaze gabbroic rocks, here we suggest the Xigaze ophiolite as a typical representative of fossil ultraslow-spreading ridges.