Cretaceous isochron ages from K–Ar and Ar / Ar dating of eclogitic rocks in the Tso Morari Complex, western Himalaya, India

The Tso Morari Complex, which is thought to be originally the margin of the Indian continent, is composed of pelitic gneisses and schists including mafic rock lenses (eclogites and basic schists). Eclogites studied here have the mineral assemblage Grt + Omp + Ca-Amp + Zo + Phn + Pg + Qtz + Rt. They also have coesite pseudomorph in garnet and quartz rods in omphacite, suggesting a record of ultrahigh-pressure metamorphism. They occur only in the cores of meter-scale mafic rock lenses intercalated with the pelitic schists. Small mafic lenses and the rim parts of large lenses have been strongly deformed to form the foliation parallel to that of the pelitic schists and show the mineral assemblages of upper greenschist to amphibolite facies metamorphism. The garnet–omphacite thermometry and the univariant reaction relations for jadeite formation give 13–21 kbar at 600 °C and 16–18 kbar at 750 °C for the eclogite formation using the jadeite content of clinopyroxene (X_Jd = 0.48).

Phengites in pelitic schists show variable Si / Al and Na / K ratios among grains as well as within single grains, and give K–Ar ages of 50–87 Ma. The pelitic schist with paragonite and phengite yielded K–Ar ages of 83.5 Ma (K = 4.9 wt.%) for paragonite–phengite mixture and 85.3 Ma (K = 7.8 wt.%) for phengite and an isochron age of 91 ± 13 Ma from the two dataset. The eclogite gives a plateau age of 132 Ma in Ar/Ar step-heating analyses using single phengite grain and an inverse isochron age of 130 ± 39 Ma with an initial ⁴⁰Ar / ³⁶Ar ratio of 434 ± 90 in Ar/Ar spot analyses of phengites and paragonites. The Cretaceous isochron ages are interpreted to represent the timing of early stage of exhumation of the eclogitic rocks assuming revised high closure temperature (500 °C) for phengite K–Ar system. The phengites in pelitic schists have experienced retrograde reaction which modified their chemistry during intense deformation associated with the exhumation of these rocks with the release of significant radiogenic ⁴⁰Ar from the crystals. The argon release took place in the schists that experienced the retrogression to upper greenschist facies metamorphisms from the eclogite facies conditions.     

Preservation/exhumation of ultrahigh-pressure subduction complexes

Ultrahigh-pressure (UHP) metamorphic terranes reflect subduction of continental crust to depths of 90–140 km in Phanerozoic contractional orogens. Rocks are intensely overprinted by lower pressure mineral assemblages; traces of relict UHP phases are preserved only under kinetically inhibiting circumstances. Most UHP complexes present in the upper crust are thin, imbricate sheets consisting chiefly of felsic units ± serpentinites; dense mafic and peridotitic rocks make up less than  10% of each exhumed subduction complex. Roundtrip prograde–retrograde P–T paths are completed in 10–20 Myr, and rates of ascent to mid-crustal levels approximate descent velocities. Late-stage domical uplifts typify many UHP complexes.

Sialic crust may be deeply subducted, reflecting profound underflow of an oceanic plate prior to collisional suturing. Exhumation involves decompression through the P–T stability fields of lower pressure metamorphic facies. Scattered UHP relics are retained in strong, refractory, watertight host minerals (e.g., zircon, pyroxene, garnet) typified by low rates of intracrystalline diffusion. Isolation of such inclusions from the recrystallizing rock matrix impedes back reaction. Thin-aspect ratio, ductile-deformed nappes are formed in the subduction zone; heat is conducted away from UHP complexes as they rise along the subduction channel. The low aggregate density of continental crust is much less than that of the mantle it displaces during underflow; its rapid ascent to mid-crustal levels is driven by buoyancy. Return to shallow levels does not require removal of the overlying mantle wedge. Late-stage underplating, structural contraction, tectonic aneurysms and/or plate shallowing convey mid-crustal UHP décollements surfaceward in domical uplifts where they are exposed by erosion. Unless these situations are mutually satisfied, UHP complexes are completely transformed to low-pressure assemblages, obliterating all evidence of profound subduction.     

Calculated Phase Relations in the System Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O with Applications to UHP Eclogites and Whiteschists

Pressure–temperature grids in the system Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O and its subsystems have been calculatedin the range 15–45 kbar and 550–900°C, usingan internally consistent thermodynamic dataset and new thermodynamicmodels for amphibole, white mica, and clinopyroxene, with thesoftware THERMOCALC. Minerals considered for the grids includegarnet, omphacite, diopside, jadeite, hornblende, actinolite,glaucophane, zoisite, lawsonite, kyanite, coesite, quartz, talc,muscovite, paragonite, biotite, chlorite, and plagioclase. Compatibilitydiagrams are used to illustrate the phase relationships in thegrids. Coesite-bearing eclogites and a whiteschist from Chinaare used to demonstrate the ability of pseudosections to modelphase relationships in natural ultrahigh-pressure metamorphicrocks. Under water-saturated conditions, chlorite-bearing assemblagesin Mg- and Al-rich eclogites are stable at lower temperaturesthan in Fe-rich eclogites. The relative temperature stabilityof the three amphiboles is hornblende > actinolite > glaucophane(amphibole names used sensu lato). Talc-bearing assemblagesare stable only at low temperature and high pressure in Mg-and Al-rich eclogites. For most eclogite compositions, talccoexists with lawsonite, but not zoisite, in the stability fieldof coesite. Water content contouring of pressure–temperaturepseudosections, along with appropriate geotherms, provides newconstraints concerning dehydration of such rocks in subductingslabs. Chlorite and lawsonite are two important H₂O-carriersin subducting slabs. Depending on bulk composition and pressure–temperaturepath, amphibole may or may not be a major H₂O-carrier to depth.In most cases, dehydration to make ultrahigh-pressure eclogitestakes place gradually, with H₂O content controlled by divariantor higher variance assemblages. Therefore, fluid fluxes in subductionzones are likely to be continuous, with the rate of dehydrationchanging with changing pressure and temperature. Further, eclogitesof different bulk compositions dehydrate differently. Dehydrationof Fe-rich eclogite is nearly complete at relatively shallowdepth, whereas Mg- and Al-rich eclogites dehydrate continuouslydown to greater depth. KEY WORDS: dehydration; eclogites; phase relations; THERMOCALC; UHP metamorphism; whiteschists     

Evolution from Oceanic Subduction to Continental Collision: a Case Study from the Northern Tibetan Plateau Based on Geochemical and Geochronological Data

Two apparently distinct, sub-parallel, paleo-subduction zonescan be recognized along the northern margin of the Tibetan Plateau:the North Qilian Suture Zone (oceanic-type) with ophioliticmélanges and high-pressure eclogites and blueschistsin the north, and the North Qaidam Belt (continental-type) inthe south, an ultrahigh-pressure (UHP) metamorphic terrane comprisingpelitic and granitic gneisses, eclogites and garnet peridotites.Eclogites from both belts have protoliths broadly similar tomid-ocean ridge basalts (MORB) or oceanic island basalts (OIB)in composition with overlapping metamorphic ages (480–440Ma, with weighted mean ages of 464 ± 6 Ma for North Qilianand 457 ± 7 Ma for North Qaidam), determined by zirconU–Pb sensitive high-resolution ion microprobe dating.Coesite-bearing zircon grains in pelitic gneisses from the NorthQaidam UHP Belt yield a peak metamorphic age of 423 ±6 Ma, 40 Myr younger than the age of eclogite formation, anda retrograde age of 403 ± 9 Ma. These data, combinedwith regional relationships, allow us to infer that these twoparallel belts may represent an evolutionary sequence from oceanicsubduction to continental collision, and continental underthrusting,to final exhumation. The Qilian–Qaidam Craton was probablya fragment of the Rodinia supercontinent with a passive marginand extended oceanic lithosphere in the north, which was subductedbeneath the North China Craton to depths >100 km at c. 423Ma and exhumed at c. 403 Ma (zircon rim ages in pelitic gneiss). KEY WORDS: HP and UHP rocks; subduction belts; zircon SHRIMP ages; Northern Tibetan Plateau     

Ca-rich Garnet-Clinopyroxene Rocks at Hujialin in the Su-Lu Terrane (Eastern China): Deeply Subducted Arc Cumulates?

Layers of Ca-rich garnet–clinopyroxene rocks enclosedin a serpentinite body at Hujialin, in the Su–Lu terraneof eastern China, preserve igneous textures, relict spinel ingarnet, and exsolution lamellae of Ca-rich garnet, ilmenite/magnetite,Fe-rich spinel, and also amphibole in clinopyroxene. In termsof their major and trace element compositions, the studied samplesform a trend from arc cumulates towards Fe–Ti gabbros.Reconstructed augite compositions plot on the trend for clinopyroxenein arc cumulates. These data suggest that the rocks crystallizedfrom mantle-derived magmas differentiated to various extentsbeneath an arc. The Ca-rich garnet + diopside assemblage isinferred to have formed by compressing Ca-rich augite, whereasthe relatively Mg-rich cores of garnet porphyroblasts may haveformed at the expense of spinel. The protolith cumulates weresubducted from near the crust–mantle boundary (c. 1 GPa)deep into the upper mantle (4·8 ± 0·6 GPaand 750 ± 50°C). Negatively sloped P–T pathsfor the garnet–clinopyroxene rocks and the corollary ofcorner flow induced subduction of mantle wedge peridotite arenot supported by the available data. Cooling with, or without,decompression of the cumulates after the igneous stage probablyoccurred prior to deep subduction. KEY WORDS: arc cumulates; Ca-rich garnet; garnet–clinopyroxene rocks; Su–Lu terrane; UHP metamorphism     

豫陕交界地区钾长石矿产地质特征及开发前景

本文研究证明区内有南、北两条钾长石成矿带，其产出形态、矿化特征、矿石类型均有差异，且导致了它们在工业应用上的不同。区内矿体以岩脉或岩株产出，有红色、浅红色、白色3种矿石。白色矿石富钾低铁，达到国标一级品，其余矿石为二级以下品级，均具有一定的销售市场。从全国的供需形势分析，今后钾长石矿产品市场前景广阔，而区内钾长石资源进一步找矿的潜力仍然很大。     

中国大陆科学钻探工程主孔100～2000 m 放射性产热元素的垂向分布特征及其成因

地球内部放射性产热元素U、Th及K(HPE)含量如何随深度的变化而变化是固体地球科学中的一个重要参数,在限定地壳的热和流变学结构、地球化学、岩石和构造模型中起关键性的作用。对HPE深部分布的认识主要来自于对大型花岗岩岩基的研究及对地表热流值和产热率之间关系的模拟,对高压-超高压变质地体的HPE随深度的分布缺乏认识。在苏鲁超高压变质带中实施的中国大陆科学钻探工程届时将提供超过5km的岩心,为确定苏鲁超高压带的HPE结构提供了最好的机会。对CCSD的100～2000m岩心的732块样品详细的放射性产热元素含量的测试及现今产热率计算的初步结果表明:(1)花岗质片麻岩具有最高的产热率,平均为1665×10-11W/kg;(2)副片麻岩(变沉积岩)具有中等的产热率,为994×10-11W/kg;(3)金红石榴辉岩及石榴石橄榄岩具有最低的产热率,为17×10-11～20×10-11W/kg;(4)放射性产热元素的浓度及相应的产热率随岩性的变化而变化,呈现阶梯状的分布特征。产热率随深度的变化特征表明CCSD主孔中的HPE三明治结构。该结构可能代表着高压-超高压变质地体中的典型HPE结构,比大型花岗岩岩基的HPE结构更复杂,与传统的HPE指数衰减分布模式不吻合。CCSD主孔中所揭示的三明治式HPE结构是大陆被动陆缘中的基性及超基性岩、沉积岩及花岗岩在深     

大别山及苏鲁地区微粒金刚石分类及其大地构造意义

1992年发现大别山首例微粒金刚石之后，又于2003年和2004年在大别山和苏鲁地区的榴辉岩薄片中和榴辉岩的人工重砂中发现了微粒金刚石。本文报道其中尚未发表的7颗，并对2颗较大的薄片中的微粒金刚石和2颗自由晶体金刚石进行拉曼光谱和红外光谱测试。研究结果表明，本区所有微粒金刚石都为IaA和IaB型金刚石的混合体。缺少Ib型金刚石，表明没有人造金刚石的混入。薄片中的金刚石大部分为石榴子石的包裹体，少数产出于颗粒之间，直径为30-180μm。自由颗粒微粒金刚石直径为400-700μm。在大别山北部，不但又一次找到了微粒金刚石，还在石榴子石中发现有单斜辉石、磷灰石和金红石的出溶。这表明北大剐不但是超高压地质体，而且可能是本区俯冲最深的地质体。     

苏鲁超高压变质岩区深部流体He-Ar的系统关系：中国大陆科学钻探工程在线流体监测的解析

中国大陆科学钻探工程在线监测从泥浆中分离出的气体地球化学组成揭示了一段重要的气体异常,从2004年12月24日夜里开始到12月29日晚上结束。从12月10日到24日晚上11点30分的气体的Ar、He和N2基本上沿着趋势A分布,而12月26日早上7点半到29日晚上7点半的数据沿趋势C分布。相对于趋势A,趋势C中的气体含有相对升高的Ar,Ar/He和Ar/N2的平均值分别为3653和0.0142,明显高于空气的比值1800和0.0119。趋势A中的气体的Ar/He和Ar/N2比值分别围绕1851和0.0118变化,其中的Ar/He比值稍微高于空气的比值,但Ar/N2比值近似于空气比值,表明背景地下流体含非常低的Ar,而He和N2主要是大气组分。在趋势A和趋势C之间的数据(时间段B)具有和空气接近的Ar/N2比值,但平均Ar/He比值为3265,明显高于空气比值,反映了该段气体具有相对亏损的He。苏鲁-大别山地区的热年代学研究已经表明云母和角闪石的Ar/Ar的冷却年龄大大地高于磷灰石或锆石的He的冷却年龄,说明Ar在超高压变质地体折返早期就已封闭,而He一直保持开放状态,直到超高压变质岩接近地表。这种Ar和He对温度变化的不同反应,导致大部分的He在超高压岩石折返过程中脱气并释放到空气中,Ar则相对圈闭在固体岩石或封闭的断裂带中。在He-Ar的系统关系上,表现为来自于超高压岩石或断裂带中的流体具有富集Ar、亏损He及升高的Ar/He比值。气体组分从趋势A向趋势C的骤然跳跃,反映了地下流体组分的强烈变化,即具有相对富集Ar的深部流体的贡献大大增强。     

台风活动对副热带高压位置和强度的影响

利用一个浅水模式，实施了6组时间积分为4－5个模式日的试验，研究了台风对副热带高压中心位置及强度的影响。结果表明：台风和副热带高压的相互作用可以使高压中心位置西移,强度增加。据此讨论了台风活动和西北地区东部干旱之间统计联系的可能机制。