西藏拉轨岗日核杂岩盖层变质分带特征及其地质意义

藏南拉轨岗日由一系列链状的热穹隆构成, 总体呈东西向延伸, 每一个热穹隆是一个变质核杂岩, 核部发育大量变质岩, 基底与盖层之间发育拆离断层.通过对拉轨岗日变质带及其特征变质矿物进行化学成分分析和温度压力估算, 得出拉轨岗日变质带的分带规律及矿物成分、变质温度、压力、深度的变化规律, 为拉轨岗日变质核杂岩的热活动提供了佐证.      

北京西山房山岩体岩浆底辟构造及其地质意义

运用底辟构造的理论和模型, 通过对北京西山房山岩体边缘围岩构造、变形和应变的研究, 厘定出岩体边缘的高温剪切带、周缘向斜、呈穹状分布陡倾的线理和面理, 并结合对西山区域构造事件分析后提出房山岩体为典型的岩浆底辟构造(HotStokesDiapir).这项研究成果不仅在世界上首次证实了岩浆底辟的存在, 而且对理清北京西山地区的地质构造格架和演化序列具有十分重要的意义.研究认为房山地区可能不存在变质核杂岩; 房山岩体边缘的关坻太古宙杂岩是基底岩石随岩体底辟流动上升带到地壳上部的; 原先确定的一些印支期“剥离断层”是房山岩体岩浆底辟的刺穿构造或围岩高温剪切作用造成的地层缺失; 太平山和凤凰山等向斜是岩体底辟过程中在围岩拖曳下形成的周缘向斜.      

酸性环境下类砂岩材料物理性质的试验研究

采用酸性环境下的加速试验方法,研究了不同时段类砂岩材料-砂浆的物理性质,测试了试样的波速,监测了溶液中H+、Ca2+浓度变化,发现在不同腐蚀阶段砂浆表现出来的物理性质相差较大,其反应过程具有阶段性和Ca2+离子溶出具有滞后性的特点。同时,对不同尺寸试样受酸腐蚀的影响程度进行了对比分析,为酸雨地区岩石和混凝土性质的研究提供了可借鉴的思路。     

苏鲁地体南缘超高压变质带朐山二云花岗片麻岩的地球化学特征及其构造意义

二云花岗片麻岩是组成苏鲁地体南缘超高压变质带朐山杂岩体的重要岩石类型,虽然经历了超高压变质作用,但仍保留了花岗岩的岩石学特征。常量元素和微量元素分析结果表明该片麻岩具有高w(K₂O)、低w(CaO)、高w(TFeO)/w(MgO)比值、铝饱和指数偏高的A型花岗岩的特征,岩石类型为高钾碱性过铝质A型花岗岩。稀土元素中轻稀土富集、分馏程度高、Eu强烈亏损,微量元素中Ba、P、Ti、Sc具有明显的负异常,w(Sr)/ w(Y)、w(La)/ w(Yb)和w(Rb)/ w(Sr)、w(Rb)/ w(Ba)较高,尤其是w(TiO₂)＜0.2％和w(Y)/ w(Nb)＞1.2的特征,以及在w(Rb)-w(Yb+Ta）和w(Rb)-w(Y+Nb）判别图解上样品投点位于板内环境区等,表明该片麻岩的原岩形成于板内与裂谷有关的非造山环境。锆石SHRIMP U-Pb测年结果揭示其侵位时代为新元古代中晚期((722±32）Ma）,与杂岩体中早期侵位的二长花岗片麻岩及杂岩体上覆地层中的变质火山岩同为Rodinia大陆裂解、扬子地块陆内裂谷形成过程中伴随的岩浆活动的产物。     

METAMORPHIC PETROLOGY

正20141408 Cai Jia(Institute of Geology,Chinese Academy of Geological Sciences,Beijing100037,China);Liu Fulai Petrogenesis and Metamorphic P-T Conditions of Garnet-Spinel-Biotitebearing Paragneiss in Danangou Area,Daqingshan-Wulashan Metamorphic Complex Belt(Acta Petrologica Sinica,ISSN1000-0569,CN11-1922/P,29(7),     

LP/HT metamorphism as a temporal marker of change of deformation style within the Late Palaeozoic accretionary wedge of central Chile

A Late Palaeozoic accretionary prism, formed at the southwestern margin of Gondwana from Early Carboniferous to Late Triassic, comprises the Coastal Accretionary Complex of central Chile (34–41°S). This fossil accretionary system is made up of two parallel contemporaneous metamorphic belts: a high‐pressure/low temperature belt (HP/LT – Western Series) and a low pressure/high temperature belt (LP/HT – Eastern Series). However, the timing of deformation events associated with the growth of the accretionary prism (successive frontal accretion and basal underplating) and the development of the LP/HT metamorphism in the shallower levels of the wedge are not continuously observed along this paired metamorphic belt, suggesting the former existence of local perturbations in the subduction regime. In the Pichilemu region, a well‐preserved segment of the paired metamorphic belt allows a first order correlation between the metamorphic and deformational evolution of the deep accreted slices of oceanic crust (blueschists and HP greenschists from the Western Series) and deformation at the shallower levels of the wedge (the Eastern Series). LP/HT mineral assemblages grew in response to arc‐related granitic intrusions, and porphyroblasts constitute time markers recording the evolution of deformation within shallow wedge material. Integrated P–T–t–d analysis reveals that the LP/HT belt is formed between the stages of frontal accretion (D₁) and basal underplating of basic rocks (D₂) forming blueschists at c. 300 Ma. A timeline evolution relating the formation of blueschists and the formation and deformation of LP/HT mineral assemblages at shallower levels, combined with published geochronological/thermobarometric/geochemistry data suggests a cause–effect relation between the basal accretion of basic rocks and the deformation of the shallower LP/HT belt. The S₂ foliation that formed during basal accretion initiated near the base of the accretionary wedge at ~30 km depth at c. 308 Ma. Later, the S₂ foliation developed at c. 300 Ma and ~15 km depth shortly after the emplacement of the granitoids and formation of the (LP/HT) peak metamorphic mineral assemblages. This shallow deformation may reflect a perturbation in the long‐term subduction dynamics (e.g. entrance of a seamount), which would in turn have contributed to the coeval exhumation of the nearby blueschists at c. 300 Ma. Finally, ⁴⁰Ar–³⁹Ar cooling ages reveal that foliated LP/HT rocks were already at ~350 °C at c. 292 Ma, indicating a rapid cooling for this metamorphic system.     

Polymetamorphic history of a relict Permian hornfels from the central Gran Paradiso Massif (Western Alps,Italy): a microstructural and thermodynamic modelling study

A petrological and thermobarometric study of the Lago Teleccio hornfelses was undertaken to reconstruct the polymetamorphic evolution and constrain the P–T conditions of Permian contact metamorphism. The Lago Teleccio metasedimentary rocks record a Variscan regional metamorphism characterized by amphibolite facies mineral assemblages including quartz, plagioclase, K‐feldspar (Kfs 1), biotite, garnet (Grt 1) and staurolite; this was followed by a late‐Variscan mylonitization event. Metamorphism of the Variscan metamorphic rocks at the contact with a Permian granitic intrusion produced static recrystallization and/or new growth of quartz, garnet (Grt 2), plagioclase, K‐feldspar (Kfs 2), cordierite, green spinel, biotite and prismatic sillimanite (Contact 1). This thermal event, which occurred at a peak pressure of 0.23–0.35 GPa, temperature of 670–700 °C and a_H2O of 0.75–1, was followed either during post‐contact metamorphism cooling or, more likely, during the early‐Alpine metamorphism by the breakdown of cordierite into an anhydrous kyanite + orthopyroxene + quartz assemblage. The poorly developed early‐Alpine eclogite facies metamorphism (Alpine 1) was characterized by relatively anhydrous mineral associations and low strain, which locally produced coronitic and pseudomorphous microstructures in metasedimentary rocks, with scanty formation of jadeite, zoisite and a new high‐pressure garnet (Grt 3). Greenschist facies retrogression (Alpine 2) was characterized by the local development of a chlorite‐ and muscovite‐bearing mineral association, suggestive of aqueous fluid incursion. In the hornfelses, the limited extent of metamorphic overprinting is suggested by the fine grain size of the Alpine mineral associations, which formed at the expense of the Permian contact metamorphic associations, and was favoured by the anhydrous mineralogy of the hornfelses.     

Crustal thinning during Mesozoic extensional detachment faulting in the Yiwulüshan region,eastern North China Craton

Two stages of extension affected the Yiwulüshan area, forming the Yiwulü High-Temperature Extensional Ductile Shear Zone (YHED) and the Waziyu Low-Temperature Extensional Ductile Shear Zone (WLED) during the Middle–Late Jurassic and Early Cretaceous, respectively. The YHED and WLED are characterized by elongation strain and plane strain, respectively. Kinematic vorticity values (W_k ), estimated from polar Mohr diagrams, suggest that pure shear-dominated and thinning-related shearing generated the YHED, whereas simple and pure shearing created the WLED during crustal thinning. From the thickness (H) and the thinning rate (μ) of the ductile shear zones, the reduced crustal thickness due to ductile shearing was estimated to be approximately 3.72 km. Based on structural analysis, contact relationships, and geochronological data, we propose that intense extensional detachment contributed to the stratigraphic gap along a Middle–Late Jurassic ductile detachment shear zone at the contact between Palaeo-Mesoproterozoic metasedimentary rocks and the Archaean basement. Furthermore, this ductile detachment shear zone was reactivated in the Early Cretaceous and lasted for 7.48 million years. After correlating the stratigraphy of the Yiwulüshan area with regions adjacent to it, we conclude that a 1.46–1.69 km-thick section of Proterozoic and Archaean basement is missing along the ductile detachment shear zone. We estimate that the crustal thickness in the Yiwulüshan region has been reduced by more than 5.41 km because of extension-related shearing and this stratigraphic gap. In addition, numerous Mesozoic extensional structures occur throughout the northeastern North China Craton, and crustal thinning has been accommodated along all of them. Our findings highlight the importance of extensional detachments and crustal thinning to lithospheric thinning.     

Geochronological and geochemical study of gneiss–schist complexes and associated granitoids,Beishan Orogen,southern Altaids

The tectonic nature of metamorphic terranes and their role in orogenesis are problematic. Here we present new U–Pb ages and geochemical data for widespread metamorphic rocks and associated granitoids from Northwest China. Orthogneisses from the metamorphic complexes have crystallization ages of ～457, ～452, and ～526 Ma. One paragneiss (schist) has a maximum depositional age of 312 ± 7 Ma. Three foliated granites were emplaced at ～450, ～349, and ～410 Ma, and all lack inherited Precambrian ages. The metamorphic terranes may have undergone multiple petrotectonic events as revealed by the metamorphic ages. Both the orthogneisses and granitoids show enrichment in large ion lithophile elements (LILEs) and light rare Earth elements (LREEs), and depletion in high field strength elements (HFSEs), which indicate that they formed in a subduction-generated accretionary arc setting. Our study demonstrates that the metamorphic terranes in the Beishan area, originally considered as Precambrian basement with suspected Neoarchaean to Palaeoproterozoic ages, are actually parts of early Palaeozoic arcs. The protoliths were probably metamorphosed arc plutonic and sedimentary rocks. Combined with other studies, we speculate that the Beishan Orogen formed by progressive arc accretion during the latest Neoproterozoic to early Palaeozoic time. This new interpretation has implications for other high-grade metamorphic terranes within orogens that have been assumed to represent ancient or pre-existing micro-continental blocks. If so, the importance of collision as a mechanism of mountain building has been overestimated, and the accretionary process as a mechanism of continental growth has been underestimated.     

Early Palaeozoic sub-arc chromitite-bearing peridotite in the Kudi ophiolite on the westernmost Tibetan Plateau

ABSTRACT

A chromite deposit was discovered in the Kudi ophiolite in the Palaeozoic western Kunlun orogenic belt. Chromite forms elongated (<2 m in width) and banded chromitite bodies (<0.1 m in width for each band) in dunite and podiform chromitite bodies (<1.5 m in width) in harzburgite. Dunite is classified into two types. Type I dunite hosting massive and banded chromitites shows low Fo in olivine (88.1–90.9), moderate Cr^# [=Cr/(Cr + Al), 0.47–0.56] in chromite, and a positively sloped primitive mantle-normalized platinum group elements (PGE) pattern, suggesting that it is a cumulate of a mafic melt. Harzburgite and type II dunite show olivine with high Fo (>91.1) and chromite with moderate to high Cr^# (0.44–0.61), and flat to negatively sloped primitive mantle-normalized PGE patterns, indicating that they are residual mantle peridotite after partial melting. Chromite in all three types of chromitites has relatively uniform moderate values Cr^# ranging from 0.43 to 0.56. Massive chromitite contains euhedral chromite with high TiO₂ (0.40–0.43 wt.%) and has a positively sloped primitive mantle-normalized PGE pattern, suggesting that it represents a cumulate of a melt. Rocks containing disseminated and banded chromite show overall low total PGE, < 117 ppb, and a negatively sloped primitive mantle-normalized PGE pattern. Chromite grains in these two types of occurrences are irregular in shape and enclose olivine grains, suggesting that chromite formed later than olivine. We suggest that chromite-oversaturated melt penetrated into the pre-existing dunite and crystallized chromite. The oxygen fugacity (fO₂ values of chromitites and peridotites are high, ranging from FMQ+0.8 (0.8 logarithmic unit above the fayalite-magnetite-quartz buffer) to FMQ+2.3 for chromitites and from FMQ+0.9 to FMQ+2.8 for peridotites (dunite and harzburgite). The mineral compositions and high fO₂ values as well as estimated parental magma compositions of the chromitites suggest that the Kudi ophiolite formed in a sub-arc setting.