蛇绿岩研究现状及华南蛇绿岩问题

蛇绿岩概念自上世纪70年代提出以来,就成为确定古板块边界的重要证据,具有重要大地构造学意义,一直是地质学家研究的热点之一。近年来,随着地球化学技术的发展以及深海钻探计划和大洋钻探计划的实施,世界各地蛇绿岩研究不断深入,取得了很大的发展,同时也提出了很多新的问题。本文主要论述了蛇绿岩研究的最新动态,并对华南蛇绿岩的研究进展进行了分析梳理,对存在的一些问题作了初步讨论。     

Structure of the Soresat Metamorphic Complex,North Sanandaj–Sirjan Zone,northwest Iran

The metasedimentary and granitoid rocks of the Soresat Metamorphic Complex occur along the northern margin of the Sanandaj–Sirjan Zone in northwest Iran. Four different deformational events (D1–D4) are recorded in the Soresat Metamorphic Complex. The D1 and D2 progressive deformation events resulted from north-northeast–south-southwest regional horizontal shortening due to the subduction of Neo-Tethys oceanic lithosphere beneath the Sanandaj–Sirjan Zone. Post-suturing convergence between Arabia and Iran, which resulted in a right lateral-reverse displacement along the suture caused the north-northwest–south-southeast horizontal shortening of D3. D4 is recorded by normal faulting. Andalusite, cordierite and sillimanite (fibrolite) record the thermal peak (with a geothermal gradient >30°C/km). Field and microscopic studies of intruded granitoid rocks in the Soresat Metamorphic Complex divide them into three major groups: (i) syn-deformation (syn-D2) granitic gneiss; (ii) late- to post-deformation (late- to post-D2) granites and granodiorites; and (iii) post-deformation (post-D2) alkali granites.     

Ophiolitic and metamorphic rocks in the Percy Isles and Shoalwater Bay region,New England Fold Belt,central Queensland

Ophiolitic and metamorphic rocks of the eastern part of the New England Fold Belt in the Shoalwater Bay region and the Percy Isles are grouped in the Marlborough and Shoalwater terranes, respectively. Marlborough terrane units occur on South Island (Percy Isles) and comprise the Northumberland Serpentinite, antigorite serpentinite with rodingite and more silicic dykes and mafic inclusions, the Chase Point Metabasalt, some 800+ metres of pillow lava, and the intervening South Island Shear Zone containing fault‐bounded slices of mafic and ultramafic igneous rocks, schist, and volcaniclastic sedimentary rocks, and zones of mélange. The Shoalwater terrane, an ancient subduction complex, consists of the Shoalwater Formation greenschist facies metamorphosed quartz sandstone and mudstone on North East Island and on the mainland at Arthur Point, the Townshend Formation, amphibolite‐grade quartzite, schist and metabasalt on Townshend Island, and the Broome Head Metamorphics on the western side of Shoalwater Bay, upper amphibolite facies quartz‐rich gneiss. With the exception of a sliver emplaced onto the western Yarrol terrane, possibly by gravity sliding, Shoalwater terrane rocks show the effects of Late Permian polyphase deformation. The Shacks Mylonite Zone along the northwest edge of the Broome Head Metamorphics marks a zone of oblique thrusting and is part of the major Stanage Fault Zone. The latter is a northeast‐striking oblique‐slip dextral tear fault active during Late Permian west‐directed thrusting that emplaced large ultramafic sheets farther south. Marlborough terrane rocks were emplaced along the Stanage Fault Zone, probably from the arc basement on which rocks of the Yarrol terrane were deposited. Structural trends and the distribution of rock units in the Shoalwater Bay‐Percy Isles region are oblique to the overall structural trend of the northern New England Fold Belt, probably due to the presence of a promontory in the convergent margin active in this region in Devonian and Carboniferous time.     

“Crack seal” vein geometry in eclogitic rocks

Abstract

Fluid-deposited veins occur in eclogite facies rocks in the Mont Viso meta-ophiolitic complex, Western Alps. These veins contain omphacite, garnet, rutile and apatite implying that veining took place at near to peak metamorphic conditions. Two types of omphacitic vein structure are described, (a) Cross-fibre veins which are charactenzed by omphacite fibres which long axis is parallel to the stretching direction of the country rocks. The fibres are cut accross by linear arrays of solid/fluid inclusions oriented normal to the long axes of the fibres, which are interpreted as crack-seal phenomena. (b) Massive, coarse-grained randomly oriented concentric and sector zoned omphacite with garnet, rutile and apatite in " hydrostatic " veins suggests nucleation in open fractures during eclogite facies metamorphic conditions.

Formation of syn-kinematic veins under conditions of at least P > 12 kbar and 500 < T < 550 °C indicates that fracturing and solution transfer processes occurred during subduction event.     

Geochemistry and petrogenesis of the Late Cretaceous Haji‐Abad ophiolite (Outer Zagros Ophiolite Belt,Iran): implications for geodynamics of the Bitlis–Zagros suture zone

The Haji‐Abad ophiolite in SW Iran (Outer Zagros Ophiolite Belt) is a remnant of the Late Cretaceous supra‐subduction zone ophiolites along the Bitlis–Zagros suture zone of southern Tethys. These ophiolites are coeval in age with the Late Cretaceous peri‐Arabian ophiolite belt including the Troodos (Cyprus), Kizildag (Turkey), Baer‐Bassit (Syria) and Semail (Oman) in the eastern Mediterranean region, as well as other Late Cretaceous Zagros ophiolites. Mantle tectonites constitute the main lithology of the Haji‐Abad ophiolite and are mostly lherzolites, depleted harzburgite with widespread residual and foliated/discordant dunite lenses. Podiform chromitites are common and are typically enveloped by thin dunitic haloes. Harzburgitic spinels are geochemically characterized by low and/or high Cr number, showing tendency to plot both in depleted abyssal and fore‐arc peridotites fields. Lherzolites are less refractory with slightly higher bulk REE contents and characterized by 7–12% partial melting of a spinel lherzolitic source whereas depleted harzburgites have very low abundances of REE and represented by more than 17% partial melting. The Haji‐Abad ophiolite crustal sequences are characterized by ultramafic cumulates and volcanic rocks. The volcanic rocks comprise pillow lavas and massive lava flows with basaltic to more‐evolved dacitic composition. The geochemistry and petrology of the Haji‐Abad volcanic rocks show a magmatic progression from early‐erupted E‐MORB‐type pillow lavas to late‐stages boninitic lavas. The E‐MORB‐type lavas have LREE‐enriched patterns without (or with slight) depletion in Nb–Ta. Boninitic lavas are highly depleted in bulk REEs and are represented by strong LREE‐depleted patterns and Nb–Ta negative anomalies. Tonalitic and plagiogranitic intrusions of small size, with calc‐alkaline signature, are common in the ophiolite complex. The Late Cretaceous Tethyan ophiolites like those at the Troodos, eastern Mediterranean, Oman and Zagros show similar ages and geochemical signatures, suggesting widespread supra‐subduction zone magmatism in all Neotethyan ophiolites during the Late Cretaceous. The geochemical patterns of the Haji‐Abad ophiolites as well as those of other Late Cretaceous Tethyan ophiolites, reflect a fore‐arc tectonic setting for the generation of the magmatic rocks in the southern branch of Neotethys during the Late Cretaceous. Copyright © 2012 John Wiley & Sons, Ltd.     

Titanium,Ti, A New Mineral Species from Luobusha,Tibet, China

We describe the new mineral species titanium,ideally Ti,found in the podiform chromitites of the Luobusha ophiolite in Tibet,People’s Republic of China.The irregular crystals range from 0.1 to 0.6 mm in diameter and form an intergrowth with coesite and kyanite.Titanium is silver grey in colour,the luster is metallic,it is opaque,the streak is grayish black,and it is non-fluorescent.The mineral is malleable,has a rough to hackly fracture and has no apparent cleavage.The estimated Mohs hardness is 4,and the calculated density is 4.503 g/cm3.The composition is Ti 99.23-100.00 wt%.The mineral is hexagonal,space group P63 /mmc.Unit-cell parameters are a 2.950(2),c 4.686(1),V 35.32(5) 3,Z = 2.The five strongest powder diffraction lines [d in(hkl)(I/I0)] are: 2.569(010)(32),2.254(011)(100),1.730(012)(16),1.478(110)(21),and 0.9464(121)(8).The species and name were approved by the CNMNC(IMA 2010–044).     

青海沱沱河乌石峰与德尔尼蛇绿岩玄武岩地球化学特征对比浅析

对乌石峰和德尔尼蛇绿岩玄武岩单元地球化学特征的对比分析,发现其形成环境存在某些共性又有区别。通过稀土配分图和构造环境判别图,初步认为乌石峰蛇绿岩代表的是不成熟洋壳环境,而德尔尼蛇绿岩代表的是典型的快速扩张的洋中脊环境,两蛇绿岩带同属古特提斯多岛洋构造体系,但其发育程度有一定差别。     

新疆西准噶尔北部谢米斯台山南坡蛇绿岩带 的发现及其意义

新疆西准噶尔和什托洛盖谷地以北,沿谢米斯台山南坡出露一套蛇绿混杂岩(查干陶勒盖蛇绿岩),岩石组合自下而上包括蛇纹石化橄榄岩、变质辉长岩、玄武岩、细碧岩及伴生的硅质岩、同源火山碎屑岩等,基质为蛇纹石化橄榄岩和绿片岩相变质玄武岩。蛇绿岩中辉长岩的LA-ICP-MS锆石U-Pb定年结果为517Ma±3Ma和519Ma±3Ma,代表了蛇绿岩的形成年龄。蛇绿岩中不同组分(橄榄岩、辉长岩和细碧岩)的稀土元素和微量元素特征,以及细碧岩的Nb/Yb-Th/Yb图解均显示该蛇绿岩为洋中脊(MOR)型。查干陶勒盖蛇绿岩与西准噶尔北部的塔尔巴哈台(库吉拜)蛇绿岩、洪古勒楞蛇绿岩,邻区东哈克斯坦北部的扎乌尔-塔金蛇绿岩和巴尔克别克蛇绿岩可以很好地进行对比连接,为成吉斯-塔尔巴哈台构造带在西准境内的时空特征提供更多的约束。     

Supra-subduction zone magmatism of the Neyriz ophiolite,Iran: constraints from geochemistry and Sr-Nd-Pb isotopes

The Neyriz ophiolite along the northeast flank of the Zagros fold-thrust belt in southern Iran is an excellent example of a Late Cretaceous supra-subduction zone (SSZ)-related ophiolite on the north side of the Neotethys. The ophiolite comprises a mantle sequence including lherzolite, harzburgite, diabasic dikes, and cumulate to mylonitic gabbro lenses, and a crustal sequence comprising a sheeted dike complex and pillow lavas associated with pelagic limestone and radiolarite. Mantle harzburgites contain less CaO and Al₂O₃, are depleted in rare earth elements, and contain spinels that are more Cr-rich than lherzolites. Mineral compositions of peridotites are similar to those of both abyssal and SSZ- peridotites. Neyriz gabbroic rocks show boninitic (SSZ-related) affinities, while crustal rocks are similar to early arc tholeiites. Mineral compositions of gabbroic rocks resemble those of SSZ-related cumulates such as high forsterite olivine, anorthite-rich plagioclase, and high-Mg# clinopyroxene. Initial εNd(t) values range from +7.9 to +9.3 for the Neyriz magmatic rocks. Samples with radiogenic Nd overlap with least radiogenic mid-ocean ridge basalts and with Semail and other Late Cretaceous Tethyan ophiolitic rocks. Initial ⁸⁷Sr/⁸⁶Sr ranges from 0.7033 to 0.7044, suggesting modification due to seafloor alteration. Most Neyriz magmatic rocks are characterized by less radiogenic ²⁰⁷Pb/²⁰⁴Pb (near the northern hemisphere reference line), suggesting less involvement of sediments in their mantle source. Our results for Neyriz ophiolite and the similarity to other Iranian Zagros ophiolites support a subduction initiation setting for its generation.     

Zircon ages and geochemical data of the Biluutiin ovoo ophiolite: implication for the tectonic evolution of South Mongolia

The early Palaeozoic tectonic evolution of South Mongolia is not well constrained due to the limited exposure of early Palaeozoic rocks in the area and the scarcity of both geochemical and geochronological data. In order to help rectify this situation, we have conducted detailed studies on the Biluutiin ovoo ophiolite in South Mongolia to provide constraints on the tectonic evolution of the region during this period. The Biluutiin ovoo ophiolite consists of ultramafic rocks, mylonitic gabbro, basalt, tuff sandstone, plagiogranite, calcite, and chert. Gabbro and plagiogranite samples from the ophiolitic complex yielded SHRIMP zircon ages of 525 ± 5 Ma and 503 ± 6 Ma, respectively. Biluutiin ovoo ophiolitic basalts display LREE and LILE enrichment and strong HFSE depletion, indicating that the ophiolite is supra-subduction zone (SSZ) type. Plagiogranite with adakite-like geochemical compositions suggests that palaeo-ocean subduction occurred in South Mongolia during Cambrian time. Intruding granite yielded a SHRIMP zircon age of 353 ± 2 Ma, indicating that the ophiolite was emplaced before early Carboniferous time. Identification of the Cambrian ophiolitic complex and the occurrence of Cambrian adakites indicate that southern Mongolia underwent a period of active volcanism during the Cambrian. The Cambrian formations are likely correlated to the early Palaeozoic subduction-accretion belt of Western Mongolia.