Ultramafic Blocks in Sumdo Region, Lhasa Block, Eastern Tibet Plateau: An Ophiolite Unit

Recently, an over 100 kin long MORB-type eclogite belt of Permian was discovered in the Sumdo (松多) region of the Lhasa block, Tibet. A critical question thus is: what is the tectonic setting of the eclogite belt and is it related to an unrecognized suture in the region? Further investigations show that there are some mafic and ultramafic rocks spatially associated with the eclogite belt in the region. Three ultramafic massifs were recognized in the Sumdo region, and called the Luomaling (罗马岭),Gongbupala (贡布爬拉) and Qiazhasumdo (卡扎松多) massifs. All the massifs are fault-contacted with greenschist (Chasagang (岔萨岗) Formation) or muscovite-quartz schist (Mabuku (马布库) Formation), and individuals are about 100 m×50 m in size extending in EW as the regional structure. All the ultramafic rocks have been entirely serpentinized, and the Gongbupala massif has been selected for study in geochemistry. Eleven chemical analyses of the rocks from the Gongbupala massif show a narrow range in contents: SiO2(35.97-40.63) wt.%, MgO (37.02-38.60) wt.%, TiO2(0.01-0.08) wt.%, Al2O3 (0.80-1.64) wt.%, (Na2O+K2O) less than 0.1 wt.%, with high volatile contents (H2O+CO2) (11.24-14.91) wt.%. After recalculation without H2O+CO2, the mean values are SiO2 45.24 wt.%, MgO 43.54 wt.%, FeOT(7.45-9.97) wt.% (8.55 wt.% in average), (MgO+FeOT) 52.09 wt.%, Mg# (100×Mg/(Mg+Fe*), where Fe* represents total Fe)=89.42-90.08, (m+f)/Si ((atomicity Mg+atomicity Fe) /atomicity Si)=1.53-1.75 (1.59 in average), respectively. The mean M/F (atomicity Mg/atomicity Fe) ratio of the rocks is 9.05, which is classified as magnesium enriched-type of ultramafic rocks. The compositional features, depleted in K, Na, Ca, Al and Ti and enriched in Mg#, indicate the characteristics of peridotite originated from a depleted mantle. The rocks have low ∑ REE with (1.60-2.68)×10-6 similar to those of the primitive mantle. The chondrite-normalized REE patterns of all samples show slightly enrichment in LREE, with (Ce/Yb)N 1.03-2.46, but a little depleted in HREE. Most samples show a slight negative anomaly in Eu, a feature In REE from a relic mantle and common features In highly serpentinized ultramafic rocks in the Yarlung-Zangbo (雅鲁藏布) ophiolite and the Bangong (班公)-Nnjiang (怒江)ophiolite in Tibet. The primitive mantle-normalized spiderdiagram of trace elements for Gongbupala ultramafic rocks yields uniform distributed pattern. They are relatively enriched in Rb, Ba, La, P element (LHSE) and depleted in Sin, Ti, Y, Yb element (HFSE), a feature of metasomatic mantle peridotite. The geochemical features of the rocks suggest that the protofith of Gongbupala serpentinite in Sumdo region is harzburgite, a typical depleted mantle rock, and may represent a dismembered ophiolite unit in the region.     

An Overview on the Composition and Age of Upper Crust of Proto-Tethyan Lajishan Intra-oceanic Arc, NE Tibet Plateau

Identification and anatomy of oceanic arcs within ancient orogenic belt are significant for better understanding the tectonic framework and closure process of paleo-ocean basin. This article summarizes the geological, geochemical, and geochronological characteristics of upper crust of Proto-Tethyan Lajishan intra-oceanic arc and provides new data to constrain the subduction evolution of the South Qilian Ocean. The intra-oceanic arc volcanic rocks, including intermediate–mafic lava, breccia, tuff, and minor felsic rocks, are distributed along southern part of the Lajishan ophiolite belt. Geochemical and isotopic compositions indicate that the intermediate–mafic lava were originated from depleted mantle contaminated by sediment melts or hydrous fluids, whereas the felsic rocks were likely generated by partial melting of juvenile mafic crust in intra-oceanic arc setting. Zircons from felsic rocks yield consistent and concordant ages ranging from 506 to 523 Ma, suggesting these volcanic rocks represent the relicts of upper crust of the Cambrian intra-oceanic arc. Combined with the Cambrian forearc ophiolite and accretionary complex, we suggest that the Cambrian intra-oceanic arc in the Lajishan ophiolite belt is belonging to the intra-oceanic arc system which was generated by south-directed subduction in the South Qilian Ocean at a relatively short interval between approximately 530 and 480 Ma.     

内蒙古迪彦庙蛇绿岩辉长岩岩石学特征

迪彦庙蛇绿岩位于中朝古板块与西伯利亚古板块之间的兴蒙造山带,主要由蛇纹石化斜辉辉橄岩、辉长岩、玄武岩、细碧角斑岩组成.岩相学研究表明,迪彦庙辉长岩分为层状辉长岩（具堆晶结构）和均质辉长岩（具辉长结构）两类,且均经历了低-中级变质改造.在详细的野外工作基础上,通过对迪彦庙辉长岩的地质特征、岩相学特征和岩石化学研究,认为其为活动大陆边缘或岛弧环境下小规模单旋回封闭系统内形成的.     

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Evolution processes of Ordovician–Devonian arc system in the South‐Kitakami Massif and its relevance to the Ordovician ophiolite pulse

The South Kitakami Massif is one of the oldest geological domains in Japan having Silurian strata with acidic pyroclastic rocks and Ordovician–Silurian granodiorite–tonalite basement, suggesting that it was matured enough to develop acidic volcanisms in the Silurian period. On the northern and western margin of the South Kitakami Massif, an Ordovician arc ophiolite (Hayachine–Miyamori Ophiolite) and high‐pressure and low‐temperature metamorphic rocks (Motai metamorphic rocks) exhumed sometime in the Ordovician–Devonian periods are distributed. Chronological, geological, and petrochemical studies on the Hayachine–Miyamori Ophiolite, Motai metamorphic rocks, and other early Paleozoic geological units of the South Kitakami Massif are reviewed for reconstruction of the South Kitakami arc system during Ordovician to Devonian times with supplementary new data. The reconstruction suggests a change in the convergence polarity from eastward‐ to westward‐dipping subduction sometime before the Late Devonian period. The Hayachine–Miyamori Ophiolite was developed above the eastward‐dipping subduction through three distinctive stages. Two separate stages of overriding plate extension inducing decompressional melting with minor involvement of slab‐derived fluid occurred before and after a stage of melting under strong influence of slab‐derived fluids. The first overriding plate extension took place in the back‐arc side forming a back‐arc basin. The second one took place immediately before the ophiolite exhumation and near the fore‐arc region. We postulate that the second decompressional melting was triggered by slab breakoff, which was preceded by slab rollback inducing trench‐parallel wedge mantle flow and non‐steady fluid and heat transport leaving exceptionally hydrous residual mantle. The formation history of the Hayachine–Miyamori Ophiolite implies that weaker plate coupling may provide preferential conditions for exhumation of very hydrous mantle. Very hydrous peridotites involved in arc magmatism have not yet been discovered except for in the Cambrian–Ordovician periods, suggesting its implications for global geodynamics, such as the thermal state and water circulation in the mantle.     

北阿尔金白尖山地区花岗闪长岩锆石U-Pb定年、Hf同位素组成及其地质意义

白尖山花岗闪长岩位于北阿尔金红柳沟-拉配泉蛇绿混杂岩带东段,呈大套岩株状产出于拉配泉岩群之中。该花岗闪长岩具有较低的SiO_2含量(62.58%～65.05%),富CaO(4.02%～4.98%),铝饱和指数A/CNK1.0(0.89～0.98),富集K、Rb、Ba,亏损Nb、Ta、Zr、Ti,属于准铝质钙碱性岩石系列,具I型花岗岩的特征;其稀土元素∑REE=90.2～137.8μg/g,具有轻重稀土元素分馏明显,轻稀土元素相对富集的特点,具弱的Eu负异常(δEu_N=0.84～0.92)。该岩石变化范围较大的锆石Hf同位素组成(ε_(Hf)(t)=-2.96～7.99)可能与源区物质不充分的岩浆混染有关,结合地球化学特征及其与实验岩石学资料的对比,其形成应为洋壳俯冲时板片脱水诱发下地壳基性岩石部分熔融产生的初始岩浆结晶作用的产物,同时在岩浆侵位过程中还受到上地壳物质(杂砂岩等)的混染。锆石U-Pb年龄为475.2±2.0Ma,结合锆石CL图像具有岩浆环带特征和Th/U值(0.30～0.75),推断该年龄为花岗闪长岩的形成年龄。综合区域地质背景,本次研究的白尖山花岗闪长岩应为红柳沟-拉配泉蛇绿混杂岩带东部地区早古生代洋壳俯冲岛弧岩浆活动的组成部分,与西部红柳沟地区(恰什坎萨依及巴什考供盆地北缘)的花岗闪长岩和石英闪长岩共同构成北阿尔金早古生代洋壳俯冲产生的岛弧岩浆岩带。此外,由西到东红柳沟-拉配泉蛇绿混杂岩带的南北两侧均发育早古生代与洋壳俯冲有关的花岗质岩石,表明整个北阿尔金洋俯冲时期可能具有双向性。     

中国西北部造山带中几个古生代俯冲增生楔的识别与确认

中国西北部涉及古亚洲和特提斯两大构造域,造山带结构复杂,成矿地质条件优越。为推进地质找矿突破行动计划,中国地质调查局在各成矿(造山)带部署了一批1∶5万、1∶25万区域地质调查与基础地质综合研究项目,取得了一批新发现、新进展,有效提升了对各成矿带成矿地质条件的认知程度,尤其是在阿尔泰南缘、南天山、南昆仑等地识别并确认出规模可观的、成矿作用优越的板块俯冲增生楔,是造山带中的增生造山亚带,是寻找斑岩型铜、构造蚀变岩型金及多金属矿的最有利区带。“增生造山带”的构造、岩浆活动及空间展布等的确认,为地质找矿突破提供了强有力的技术支撑。     

Platinum-group elements distribution and spinel composition in podiform chromitites and associated rocks from the upper mantle section of the Neoproterozoic Bou Azzer ophiolite, Anti-Atlas, Morocco

The distribution of platinum-group elements (PGEs), together with spinel composition, of podiform chromitites and serpentinized peridotites were examined to elucidate the nature of the upper mantle of the Neoproterozoic Bou Azzer ophiolite, Anti-Atlas, Morocco. The mantle section is dominated by harzburgite with less abundant dunite. Chromitite pods are also found as small lenses not exceeding a few meters in size. Almost all primary silicates have been altered, and chromian spinel is the only primary mineral that survived alteration. Chromian spinel of chromitites is less affected by hydrothermal alteration than that of mantle peridotites. All chromitite samples of the Bou Azzer ophiolite display a steep negative slope of PGE spidergrams, being enriched in Os, Ir and Ru, and extremely depleted in Pt and Pd. Harzburgites and dunites usually have intermediate to low PGE contents showing more or less unfractionated PGE patterns with conspicuous positive anomalies of Ru and Rh. Two types of magnetite veins in serpentinized peridotite, type I (fibrous) and type II (octahedral), have relatively low PGE contents, displaying a generally positive slope from Os to Pd in the former type, and positive slope from Os to Rh then negative from Rh to Pd in the latter type. These magnetite patterns demonstrate their early and late hydrothermal origin, respectively. Chromian spinel composition of chromitites, dunites and harzburgites reflects their highly depleted nature with little variations; the Cr# is, on average, 0.71, 0.68 and 0.71, respectively. The TiO₂ content is extremely low in chromian spinels, <0.10, of all rock types. The strong PGE fractionation of podiform chromitites and the high-Cr, low-Ti character of spinel of all rock types imply that the chromitites of the Bou Azzer ophiolite were formed either from a high-degree partial melting of primitive mantle, or from melting of already depleted mantle peridotites. This kind of melting is most easily accomplished in the supra-subduction zone environment, indicating a genetic link with supra-subduction zone magma, such as high-Mg andesite or arc tholeiite. This is a general feature in the Neoproterozoic upper mantle.     

新疆东准噶尔锡矿北花岗斑岩的锆石LA-ICP-MS U-Pb测年

对东准噶尔锡矿北花岗斑岩进行锆石LA-ICP-MS U-Pb测年,获得206Pb/238U加权平均年龄为(281±10)Ma,MSWD=9.3,206Pb/238U-207Pb/235U谐和曲线图中下交点年龄为(278±11)Ma,MSWD=7.5,两者在误差范围内完全一致,时代属于早二叠世。结果表明,锡矿北花岗斑岩形成的时代属于东准噶尔后碰撞深成岩浆活动的范围(330～265Ma),晚于东准噶尔乌伦古河碱性花岗岩和卡拉麦里碱性花岗岩的形成时代（300Ma左右）,以花岗斑岩为代表的晚古生代岩浆侵入活动延续到早二叠世晚期。     

西藏普兰超镁铁岩体东部铬尖晶石矿物学特征及其地质意义

尖晶石是地幔橄榄岩中一种非常重要的矿物,虽然含量很低,却可以作为其寄主岩石——地幔橄榄岩——的成因指示剂.普兰岩体东部铬尖晶石分布广泛,几乎在所有岩石类型中都有出现.通过矿物学特征的研究,认为尖晶石可分为两类,一类较自形,颜色较深;另一类多呈不规则状和蠕虫状,颜色较浅.根据其化学特征,又可分为富铬型的尖晶石(Cr#＞60)和富铝型的尖晶石(Cr#＜60),富铬型尖晶石主要出现在纯橄岩、蛇纹岩和辉石岩中,富铝型尖晶石主要出现在二辉橄榄岩中.其中,方辉橄榄岩中的尖晶石形态、化学成分变化比较大,说明它经历了较为复杂的形成和变化过程.通过对尖晶石矿物学和化学特征的综合研究,认为该区尖晶石具有深海橄榄岩和上部俯冲板片(SSZ)构造环境橄榄岩特征,普兰岩体可能先后经历了MOR和SSZ两种构造环境.