Reaction textures in scapolite–wollastonite–grossular calc-silicate rock from the Kerala Khondalite Belt, Southern India: evidence for high-temperature metamorphism and initial cooling

Scapolite–wollastonite–grossular bearing calc-silicate rocks from the Vellanad area in the Kerala Khondalite Belt (KKB) of Southern India preserve a number of reaction textures which help to deduce their P–T–fluid history. Textures include calcite+plagioclase±quartz symplectites after scapolite, grossular+quartz coronas between wollastonite and plagioclase, grossular coronas between wollastonite and plagioclase+calcite that replace former scapolite, and grossular blebs replacing anorthite+calcite+quartz pseudomorphs of scapolite. Garnet coronas are also observed between clinopyroxene and wollastonite or scapolite or plagioclase. The reactions, apart from those involving clinopyroxene, can be modelled in the simple CaO–Al₂O₃–SiO₂–CO₂ system and interpreted using partial reaction grids constructed for the activities of end-members in the analysed phases. The reaction topologies produced are good approximations for the peak as well as retrograde mineral assemblages and reaction textures. For the compositions of the phases present in this study, the medium pressure calc-silicate assemblages are defined by the stable pseudo-invariant points [Qtz], [Mei] and [Grs]. The textural features interpreted using these activity-corrected grids indicate a phase of isobaric cooling from about 835°C to 750°C at 6 kbar in the Vellanad area. This is inconsistent with earlier studies on other lithologies from the KKB, most of which imply a post-peak P–T path involving near-isothermal decompression. However, as the temperatures obtained for the KKB from the calc-silicates are higher than those previously deduced from metapelites and garnet–orthopyroxene assemblages, the phase of near-isobaric cooling reported here is inferred to have proceeded prior to the onset of the decompression documented from studies of other rock types.     

五台山新太古代绿岩带中高镁安山岩（HMA）的化学特征

采用高镁安山岩（HMA）新的分类方法,描述了五台地区新太古代绿岩带中HMA的化学特征,并与实验的HMA以及自然界中典型的HMA（日本的Chichijima地区HMA和Setouchi地区HMA）进行了对比,显示该区HMA具有与上述HMA类似的化学特征。初步认为五台新太古代HMA产于洋俯冲带的环境,进而表明板块构造理论的适用时间范围可推演到新太古代。     

中国南天山晚石炭世—早二叠世花岗质侵入岩的岩石成因与地壳增生

晚石炭世—早二叠世在中亚南天山造山带形成了大量的花岗质侵入岩.中国境内这些岩体以晚石炭世Ⅰ型花岗岩、早二叠世Ⅰ型和S型花岗岩以及最晚期的A型花岗岩为代表.不同类型的岩石在源区特征和岩浆形成的温压条件上存在一定的差异,而这些差异性反映了该时期内构造环境的演化过程.本文选取铁列克岩体、盲起苏岩体、英买来岩体、川乌鲁杂岩体、...     

The geological development of the southern part of the New England Fold Belt

Two major divisions of the New England Fold Belt, Zone A and Zone B, are separated by the Peel Fault. Deposition in these two zones was probably contemporaneous (Lower Palaeozoic ‐ Lower Permian). Terminal orogenesis in both zones was also contemporaneous (Middle Permian) but whereas in Zone A deformation was only moderate, metamorphism was of burial type, and granitic emplacement was uncommon, in Zone B many rocks were severely deformed and regionally metamorphosed, and both syn‐tectonic and post‐tectonic granites are widespread.

Pre‐orogenic palaeogeography is envisaged in terms of an evolving volcanic chain ‐ fore‐chain basin ‐ trench system, with an outer non‐volcanic arc developed in the Carboniferous. Cessation of movement on a subduction zone dipping westward beneath the volcanic chain is believed to have caused the Middle Permian deformation, but neither metamorphism nor the granitic rocks are directly related to subduction.     

论博格达俯冲撕裂型裂谷的形成与演化

博格达裂谷带位于准噶尔与吐－哈两个前寒武纪地块之间,呈东－西走向,东端与克拉麦里－哈尔里克泥盆－石炭纪火山弧呈大角度相交。该 裂谷于早－中石炭世启动和沉降,在盆地中堆积了巨厚的陆源碎屑岩夹双峰式火山岩。裂谷的闭合发生于中石炭世末至晚石炭世。在裂谷闭合后区域构造由挤压向拉张的转折时期,发生了以辉绿岩为主的侵入活动,并伴有少量中－酸性分异产物。博格达裂谷东、西两段的演化特征有着显著差异。东段早石炭世就已开始裂离,裂离过程的火山岩以玄武岩为主,仅有少量流纹岩,裂谷盆地强烈沉降,形成深海－半深海环境,裂谷在中石炭世末至晚石炭世初即已闭合,裂谷岩系因强烈褶皱,与上覆二叠系呈明显角度不整合,显示了“突变”式闭合特征。与此不同的是,西段至中石炭世才开始明显裂离,裂离过程的火山岩以英安岩和流纹岩为主,玄武岩量较少,火山－沉积岩系均形成于浅海环境,裂谷至晚石炭世末才发生闭合,裂谷岩系因未发生强烈褶皱,故与上覆二叠系为平行不整合接触,显示了“渐变”式闭合特征。该裂谷的形成是古亚洲洋壳向先存的准－吐－哈陆块斜向俯冲,将其东南端撕裂的产物,因而可称为俯冲撕裂型裂谷。演化过程沿走向的明显不均一性是这类裂谷的重要特点。     

Discovery of a binary Centaur

We have identified a binary companion to (42355) 2002 CR₄₆ in our ongoing deep survey using the Hubble Space Telescope's High Resolution Camera. It is the first companion to be found around an object in a non-resonant orbit that crosses the orbits of giant planets. Objects in orbits of this kind, the Centaurs, have experienced repeated strong scattering with one or more giant planets and therefore the survival of binaries in this transient population has been in question. Monte Carlo simulations suggest, however, that binaries in (42355) 2002 CR₄₆-like heliocentric orbits have a high probability of survival for reasonable estimates of the binary's still-unknown system mass and separation. Because Centaurs are thought to be precursors to short period comets, the question of the existence of binary comets naturally arises; none has yet been definitively identified. The discovery of one binary in a sample of eight observed by HST suggests that binaries in this population may not be uncommon.     

Evolution of regional metamorphism in the Cape Smith Thrust Belt (northern Quebec, Canada): interaction of tectonic and thermal processes

Abstract Syn-metamorphic re-imbrication of the internal part of thrust belts can result in distinct pressure–temperature–time–deformation ( P–T–t–d ) pathways for different structural–metamorphic domains. In the early Proterozoic Cape Smith Thrust Belt (Canada), an external (piggyback-sequence thrusting) domain is characterized by thermal peak metamorphism occurring after deformation. In contrast, thermal peak metamorphism in an internal domain occurred during re-imbrication by out-of-sequence thrusting. The interactions of tectonic and thermal processes have been studied using three methods: (i) qualitative evaluation of the timing between mineral growth and deformation; (ii) analytical P–T paths from growth-zoned garnet porphyroblasts; and (iii) numerical modelling of vertical heat conduction. Derived P–T–t–d pathways suggest that uplift in the external domain resulted in part from erosion and isostatic unloading. In contrast, paths for the internal domain indicate that the out-of-sequence portion of the thrust belt may have experienced faster unroofing relative to the external domain. This is attributed to thickening by out-of-sequence thrusting and possibly to extensional faulting at (now eroded) higher structural levels. Observations on the timing of metamorphism, coupled with numerical modelling, suggest that the thermal peak metamorphism documented in the external domain is a consequence of the emplacement of the out-of-sequence thrusts stack in the internal portion of the thrust belt.     

Silurian A-type granitoids in the southern margin of the Tongbai-Dabieshan: Evidence from SHRIMP zircon geochronology and geochemistry

The Dabieshan Orogenic belt is well known for the exhumation of early Mesozoic ultrahigh-pressure (UHP) metamorphic rocks and Jurassic–Cretaceous emplacement of voluminous granitoids. However, the tectonic evolution in the orogen during the Paleozoic, especially its magmatic response to tectonism has not received much attention. As indicated by published data, the Dabieshan orogenic belt contains different records of Paleozoic magmatic-tectonic association in different tectonic units. Occ…     

Evolution of a complex isolated dome system,Cerro Pizarro,central México

Cerro Pizarro is an isolated rhyolitic dome in the intermontane Serdán-Oriental basin, located in the eastern Trans-Mexican Volcanic Belt. Cerro Pizarro erupted ~1.1 km³ of magma at about 220 ka. Activity of Cerro Pizarro started with vent-clearing explosions at some depth; the resultant deposits contain clasts of local basement rocks, including Cretaceous limestone, ~0.46-Ma welded tuff, and basaltic lava. Subsequent explosive eruptions during earliest dome growth produced an alternating sequence of surge and fallout layers from an inferred small dome. As the dome grew both vertically and laterally, it developed an external glassy carapace due to rapid chilling. Instability of the dome during emplacement caused the partial gravitational collapse of its flanks producing various block-and-ash-flow deposits. After a brief period of repose, re-injection of magma caused formation of a cryptodome with pronounced deformation of the vitrophyric dome and the underlying units to orientations as steep as near vertical. This stage began apparently as a gas-poor eruption and no explosive phases accompanied the emplacement of the cryptodome. Soon after emplacement of the cryptodome, however, the western flank of the edifice catastrophically collapsed, causing a debris avalanche. A hiatus in eruptive activity was marked by erosion of the cone and emplacement of ignimbrite derived from a caldera to the north of Cerro Pizarro. The final growth of the dome growth produced its present shape; this growth was accompanied by multiple eruptions producing surge and fallout deposits that mantle the topography around Cerro Pizarro. The evolution of the Cerro Pizarro dome holds aspects in common with classic dome models and with larger stratovolcano systems. We suggest that models that predict a simple evolution for domes fail to account for possibilities in evolutionary paths. Specifically, the formation of a cryptodome in the early stages of dome formation may be far more common than generally recognized. Likewise, sector collapse of a dome, although apparently rare, is a potential hazard that must be recognized and for which planning must be done.Editorial responsibility: J. Gilbert