南天山榆树沟—铜花山构造混杂体雏议

榆树沟—铜花山构造混杂体,东西长大于70km,南北宽约25km,是由不同规模的外来和原地岩块组成的。蛇绿岩套则强烈地混杂在变质基质之中,它们通过韧性剪切带或断裂与不同时代的外来岩块相接触,而它们之间无任何热接触现象。蛇绿岩套层序由上而下是沉积岩、火山岩、火成堆积岩和地幔橄榄岩。根据岩石学特征,组成这些外来和原地岩块的奥陶系、志留系、泥盆系和蛇绿岩,形成于弧-盆和活动大陆边缘环境。构造混杂体形成一系列北西西向推覆构造。蛇绿岩与原地岩块通常构成向南和向北推覆的低角度冲掩断层。晚期推覆构造使其更加复杂化。     

蛇绿岩型金刚石和铬铁矿深部成因

地球上的原生金刚石主要有3种产出类型,分别来自大陆克拉通下的深部地幔金伯利岩型金刚石、板块边界深俯冲变质岩中超高压变质型金刚石,和陨石坑中的陨石撞击型金刚石。在全球5个造山带的10处蛇绿岩的地幔橄榄岩或铬铁矿中均发现金刚石和其他超高压矿物的基础上,我们提出地球上一种新的天然金刚石产出类型,命名为蛇绿岩型金刚石。认为蛇绿岩型金刚石普遍存在于大洋岩石圈的地幔橄榄岩中,并提出蛇绿岩型金刚石和铬铁矿的深部成因模式。认为早期俯冲的地壳物质到达地幔过渡带(410~660 km深度)后被肢解,加入到周围的强还原流体和熔体中,当熔融物质向上运移到地幔过渡带顶部,铬铁矿和周围的地幔岩石以及流体中的金刚石等深部矿物一并结晶,之后,携带金刚石的铬铁矿和地幔岩石被上涌的地幔柱带至浅部,经历了洋盆的拉张和俯冲阶段,最终在板块边缘就位。     

Study on the Tectonic Setting for the Ophiolites in Xigaze, Tibet

The Xigaze ophiolite is located in the middle section of the Yarlung Zangbo River ophiolite belt and includes a well-preserved sequence section of seven ophiolite blocks. The relatively complete ophiolitic sequence sections are represented by Jiding, Dejixiang, Baigang, and Dazhuqu ophiolites and consist of three–four units. The complete ophiolite sequence in order from the bottom to top consists of mantle peridotite, cumulates, sheeted sill dike swarms, and basic lavas±radiolarian chert. These cumulates are absent in the remaining blocks of Dejixiang and Luqu. The age of radiolaria in the radiolarian chert is Late Jurassic–Cretaceous. The basalt and ultramafic rock of the ophiolite also are overlaid by Tertiary Liuqu conglomerate, which contains numerous pebble components of ophiolite, indicating that the Tethys Ocean began to close at the end of Cretaceous Period. The isotopic data of gabbro, diabase, and albite granite in the Xigaze ophiolite are approximately 126–139 Ma, which indicates that the ophiolite formed in the Early Cretaceous. The K–Ar age of amphibole in garnet amphibolite in the ophiolite mélange is 81 Ma, indicating that tectonic ophiolite emplacement occurred at the end of Late Cretaceous.     

The Nature of the Crustal Structure of the Eastern Anatolian Plateau,Turkey

The Eastern Anatolian Plateau (EAP) of Turkey, with an elevation ranging from 1700 to 2000 m, is located between the Eastern Pontide Arc to the north and the Arabian Platform to the south. In this region, pre-Maastrichtian tectonic units representing the crust crop out in only a few localities. As they are covered by Maastrichtian-Quaternary rock units, it is difficult to study the nature and mutual relationships of these pre-Maastrichtian tectonic units.

The palaeotectonic units of the EAP comprise two different levels in the present study: (1) The lower level consists of platform-type carbonates and their metamorphic equivalents. These units may represent the Taurus Platform and its metamorphic equivalents. (2) The upper level consists of an ophiolitic-mélange prism which is made up mainly of oceanic crust; the prism comprises a complex of ophiolite, ophiolitic mélange, and fore-arc deposits. This upper unit represents a subduction-accretion prism and may have originated partly from the North Anatolian Suture to the north, and partly from the South-eastern Anatolian Suture to the south.

Continental crustal rocks were thrust over by the ophiolitic mélange prism; thus outcrops of them are scarce in the region as they are exposed in tectonic windows through the ophiolitic thrust sheets. The pre-Maastrichtian tectonic units of the EAP are blanketed by Maastrichtian to Quaternary volcanic and sedimentary rock units; these sequences include successive transgressive and regressive intervals and overlie the palaeotectonic units along a pronounced unconformity. Olistostromal units are abundant in the Eocene sedimentary units and were derived from the ophiolites and ophiolitic mélange. The Maastrichtian-Quaternary cover is made up of collisional and post-collisional deposits across the whole region.

Although the EAP has been experiencing considerable N-S compression, it has not been affected by significant crustal thickening because of the strike-slip tectonic regime that is dominant in the region.     

Linking the NE Anatolian and Lesser Caucasus ophiolites: evidence for large-scale obduction of oceanic crust and implications for the formation of the Lesser Caucasus-Pontides Arc

In the Lesser Caucasus and NE Anatolia, three domains are distinguished from south to north: (1) Gondwanian-derived continental terranes represented by the South Armenian Block (SAB) and the Tauride–Anatolide Platform (TAP), (2) scattered outcrops of Mesozoic ophiolites, obducted during the Upper Cretaceous times, marking the northern Neotethys suture, and (3) the Eurasian plate, represented by the Eastern Pontides and the Somkheto-Karabagh Arc. At several locations along the northern Neotethyan suture, slivers of preserved unmetamorphozed relics of now-disappeared Northern Neotethys oceanic domain (ophiolite bodies) are obducted over the northern edge of the passive SAB and TAP margins to the south. There is evidence for thrusting of the suture zone ophiolites towards the north; however, we ascribe this to retro-thrusting and accretion onto the active Eurasian margin during the latter stages of obduction. Geodynamic reconstructions of the Lesser Caucasus feature two north dipping subduction zones: (1) one under the Eurasian margin and (2) farther south, an intra-oceanic subduction leading to ophiolite emplacement above the northern margin of SAB. We extend our model for the Lesser Caucasus to NE Anatolia by proposing that the ophiolites of these zones originate from the same oceanic domain, emplaced during a common obduction event. This would correspond to the obduction of non-metamorphic oceanic domain along a lateral distance of more than 500?km and overthrust up to 80?km of passive continental margin. We infer that the missing volcanic arc, formed above the intra-oceanic subduction, was dragged under the obducting ophiolite through scaling by faulting and tectonic erosion. In this scenario part of the blueschists of Stepanavan, the garnet amphibolites of Amasia and the metamorphic arc complex of Erzincan correspond to this missing volcanic arc. Distal outcrops of this exceptional object were preserved from latter collision, concentrated along the suture zones.     

Geology of southeast Bohol,central Philippines: Accretion and sedimentation in a marginal basin

Recent field mapping has refined our understanding of the stratigraphy and geology of southeastern Bohol, which is composed of a Cretaceous basement complex subdivided into three distinct formations. The basal unit, a metamorphic complex named the Alicia Schist, is overthrust by the Cansiwang mélange, which is, in turn, structurally overlain by the Southeast Bohol Ophiolite Complex. The entire basement complex is overlain unconformably by a ～2000 m thick sequence of Lower Miocene to Pleistocene carbonate and clastic sedimentary rocks and igneous units. Newly identified lithostratigraphic units in the area include the Cansiwang mélange, a tectonic mélange interpreted as an accretionary prism, and the Lumbog Volcaniclastic Member of the Lower Miocene Carmen Formation. The Cansiwang mélange is sandwiched between the ophiolite and the metamorphic complex, suggesting that the Alicia Schist was not formed in response to emplacement of the Southeast Bohol Ophiolite Complex. The accretionary prism beneath the ophiolite complex and the presence of boninites suggest that the Southeast Bohol Ophiolite Complex was emplaced in a forearc setting. The Southeast Bohol Ophiolite Complex formed during the Early Cretaceous in a suprasubduction zone environment related to a southeast‐facing arc (using present‐day geographical references). The accretion of this ophiolite complex was followed by a period of erosion and then later by extensive clastic and carbonate rock deposition (Carmen Formation, Sierra Bullones Limestone and Maribojoc Limestone). The Lumbog Volcaniclastic Member and Jagna Andesite document intermittent Tertiary volcanism in southeastern Bohol.     

Geochemistry of hornblende gabbros from Sonidzuoqi,Inner Mongolia,North China: implications for magmatism during the final stage of suprasubduction‐zone ophiolite formation

Allochthonous hornblende‐rich gabbroic rocks at Sonidzuoqi constitute important components of the early to middle Palaeozoic orogen, which forms the southeastern part of the Central Asian orogenic belt in Inner Mongolia. Limited hornblende K–Ar and SHRIMP U–Pb zircon ages document the Late Silurian to Early Devonian gabbroic emplacement. The rocks are tholeiitic and are characterized by moderate large‐ion‐lithophile‐element (e.g. Th, U) abundances, high‐field‐strength‐element (e.g. Nb, Ta, Zr, Ti) depletions, high Ti/V ratios, and MORB‐like isotopic signatures [(⁸⁷Sr/⁸⁶Sr)_i≈0.7030 to 0.7042; ε_Nd(t)≈+4.35 to +7.80, (²⁰⁶Pb/²⁰⁴Pb)_i≈17.46 to 17.61]. These features argue for a hydrous basaltic parental magma. We postulate that the melt formed through the coupling of MORB‐type mantle upwelling with aqueous fluid influx derived from slab devolatilization. This petrogenetic scenario suggests that an active spreading centre entered the trench during ridge subduction, bringing to a close an episode of suprasubduction‐zone ophiolite formation. The Siluro‐Devonian hornblende gabbros, together with a pre‐490 Ma ophiolitic mélange of MORB‐OIB affinity, ～483–471 Ma arc intrusions, ～498–461 Ma trondhjemite‐tonalite‐granodiorite plutons, and ～427–423 Ma calc‐alkaline granites from the same area, provide documentation of multistage crustal generation processes during the life cycle of this suprasubduction‐zone ophiolite.     

Petrology,geochemistry, and geochronology of mafic rocks from the Taoxinghu Devonian ophiolite,LongmuCo–Shuanghu–Lancang suture zone,northern Tibet: evidence for an intra-oceanic arc–basin system

Abstract

A newly discovered Devonian ophiolite located in the Taoxinghu area of central Qiangtang on the Qinhai–Tibet Plateau is described. The ophiolite consists of gabbro and diabasic dikes, and invasive cumulate gabbros-leucogabbros. The ophiolite has undergone greenschist facies metamorphism and minor deformation. Dating of the metagabbro by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) zircon U–Pb techniques yielded a weighted mean age of 367.2 ± 3.3 Ma (Late Devonian). Whole-rock geochemical analyses show that the rocks belong to the tholeiite series, with weak depletion in light rare-earth elements (LREEs), almost no Eu anomalies, weak enrichment in large-ion lithophile elements (LILEs), depletion in Nb and Ta, and weak negative Hf and Ti anomalies. These characteristics are similar to those of back-arc basin basalts. Together, these characteristics suggest that the rocks of the Devonian ophiolite formed by ~30% partial melting of spinel lherzolite, which was enriched by interaction with aqueous fluids during the late-generation phases; there is no evidence of subduction-related melting. The Devonian ophiolite rocks in the Taoxinghu area were first discovered in the LongmuCo–Shuanghu–Lancang suture zone. Detailed geochemical analyses show that the rocks formed in a back-arc ocean basin environment, indicating that the LongmuCo–Shuanghu–Lancang suture zone in central Qiangtang represents a Late Devonian intra-oceanic subduction zone in the Palaeo–Tethys Ocean. The discovery of the central Qiangtang Devonian ophiolite provides essential data for understanding the evolutionary history of the Palaeo–Tethys Ocean, and for identifying and understanding the roles of the different tectonic units on the Qinghai–Tibet Plateau.     

Geodynamic implications of ophiolitic chromitites in the La Cabaña ultramafic bodies,Central Chile

Chromitites (>80% volume chromite) hosted in two ultramafic bodies (Lavanderos and Centinela Bajo) from the Palaeozoic metamorphic basement of the Chilean Coastal Cordillera were studied in terms of their chromite composition, platinum-group element (PGE) abundances, and Re-Os isotopic systematics. Primary chromite (Cr# = 0.64–0.66; Mg# = 48.71–51.81) is only preserved in some massive chromitites from the Centinela Bajo ultramafic body. This chemical fingerprint is similar to other high-Cr chromitites from ophiolite complexes, suggesting that they crystallized from arc-type melt similar to high-Mg island-arc tholeiites (IAT) and boninites in supra-subduction mantle. The chromitites display enrichment in IPGE (Os, Ir, Ru) over PPGE (Rh, Pt, Pd), with PGE concentrations between 180 and 347 ppb, as is typical of chromitites hosted in the mantle of supra-subduction zone (SSZ) ophiolites. Laurite (RuS₂)-erlichmanite (OsS₂) phases are the most abundant inclusions of platinum-group minerals (PGM) in chromite, indicating crystallization from S-undersaturated melts in the sub-arc mantle. The metamorphism associated with the emplacement of the ultramafic bodies in the La Cabaña has been determined to be ca. 300 Ma, based on K-Ar dating of fuchsite. Initial ¹⁸⁷Os/¹⁸⁸Os ratios for four chromitite samples, calculated for this age, range from 0.1248 to 0.1271. These isotopic compositions are well within the range of chromitites hosted in the mantle section of other Phanaerozoic ophiolites. Collectively, these mineralogical and geochemical features are interpreted in terms of chromite crystallization in dunite channels beneath a spreading centre that opened a marginal basin above a supra-subduction zone. This implies that chromitite-bearing serpentinites in the metamorphic basement of the Coastal Cordillera are of oceanic-mantle origin and not oceanic crust as previously suggested. We suggest that old subcontinental mantle underlying the hypothetical Chilenia micro-continent was unroofed and later altered during the opening of the marginal basin. This defined the compositional and structural framework in which the protoliths of the meta-igneous and meta-sedimentary rocks of the Eastern and Western Series of the Chilean Coastal Cordillera basement were formed.