Alpine and Pacific styles of Phanerozoic mountain building: subduction-zone petrogenesis of continental crust

A broad continuum exists between two distinct end-member types of mountain building. Alpine-type orogenic belts develop during subduction of an ocean basin between two continental blocks, resulting in collision. They are characterized by an imbricate sequence of oceanward verging nappes; some Alpine belts exhibit superimposed late-stage backthrusting. Sediments are chiefly platform carbonates and siliciclastics, in some cases associated with minor amounts of bimodal volcanics; pre-existing granitic gneisses and related continental rocks constitute an autochthonous–parautochthonous basement. Metamorphism of deeply subducted portions of the orogen ranges from relatively high-pressure (HP) to ultrahigh-pressure (UHP). Calcalkaline volcanic–plutonic rocks are rare, and have peraluminous, S-type bulk compositions. In contrast, Pacific-type orogens develop within and landward from long-sustained oceanic subduction zones. They consist of an outboard oceanic trench–accretionary prism, and an inboard continental margin–island arc. The oceanic assemblage consists of first-cycle, in-part mélanged volcaniclastics, and minor but widespread cherts ± deep-water carbonates, intimately mixed with disaggregated ophiolites. The section recrystallized under HP conditions. Recumbent fold vergence is oceanward. A massive, slightly older to coeval calcalkaline arc is sited landward from the trench complex on the stable, non-subducted plate. It consists of abundant, dominantly intermediate, metaluminous, I-type volcanics resting on old crust; both assemblages are thrown into open folds, intruded by comagmatic I-type granitoids, and metamorphosed locally to regionally under high-T, low-P conditions. In the subduction channel of collisional and outboard Circumpacific terranes, combined extension above and subduction below allows buoyancy-driven ascent of ductile, thin-aspect ratio slices of HP–UHP complexes to midcrustal levels, where most closely approached neutral buoyancy; exposure of rising sheets caused by erosion and gravitational collapse results in moderate amounts of sedimentary debris because exhumed sialic slivers are of modest volume. At massive sialic buildups associated with convergent plate cuSPS (syntaxes), tectonic aneurysms may help transport HP–UHP complexes from mid- to upper-crustal levels. The closure of relatively small ocean basins that typify many intracratonic suture zones provides only limited production of intermediate and silicic melts, so volcanic–plutonic belts are poorly developed in Alpine orogens compared with Circumpacific convergent plate junctions. Generation of a calcalkaline arc mainly depends on volatile evolution at the depth of magma generation. Phase equilibrium studies show that, under typical subduction-zone P–T trajectories, clinoamphibole ± Ca–Al hydrous silicates constitute the major hydroxyl-bearing phases in deep-seated metamorphic rocks of MORB composition; other hydrous minerals are of minor abundance. Ca and Na clinoamphiboles dehydrate at pressures of above approximately 2 GPa, but low-temperature devolatilization may be delayed by pressure overstepping; thus metabasaltic blueschists and amphibolites expel H₂O at melt-generation depths, and commonly achieve stable eclogitic assemblages. Partly serpentinized mantle beneath the oceanic crust dehydrates at roughly comparable conditions. For reasonable subduction-zone geothermal gradients however, white micas ± biotites remain stable to pressures >3 GPa. Accordingly, attending descent to depths of >100 km, mica-rich quartzofeldspathic lithologies that constitute much of the continental crust fail to evolve substantial amounts of H₂O, and transform incompletely to stable eclogite-facies assemblages. Underflow of amphibolitized oceanic lithosphere thus generates most of the deep-seated volatile flux, and the consequent partial melting to produce the calcalkaline suite, along and above a subduction zone; where large volumes of micaceous intermediate and felsic crustal materials are carried down to great depths, volatile flux severely diminishes. Thus, continental collision in general does not produce a volcanic–plutonic arc whereas in contrast, the long-continued contemporaneous underflow of oceanic lithosphere does.     

Overview of UHP Metamorphism and Tectonics in Well-Studied Collisional Orogens

In Eurasia, the Qinling-Dabie-Sulu belt of eastern China, the Kokchetav Complex of northern Kazakhstan, the Maksyutov Complex of the southern Urals, the Dora-Maira massif of the Western Alps, and the Western Gneiss Region of Norway mark profound intracontinental collisional sutures. Adjacent regions exhibit scant evidence of contemporaneous calc-alkaline volcanism/plutonism. Each ultrahigh-pressure (UHP) metamorphic complex contains mineralogic and textural relics of coesite ± diamond as well as other very high P, moderate-T phases such as K-rich clinopyroxene, Mg-rich garnet, ellenbergerite, lawsonite, Al-rutile, glaucophane, high-Si phengite, and the phase assemblages coesite + dolomite, magnesite + diopside, and talc + kyanite, diopside, jadeite, or phengite. In each of these well-studied Eurasian complexes, maximum pressures approached or exceeded 2.8 GPa. Deep-seated recrystallization of old, cool continental crust took place during Phanerozoic time. Subduction zones constitute the only known plate-tectonic environment where such high-P, low-T conditions exist. Disaggregated, exhumed ultrahigh-pressure terranes consist of relatively thin sialic sheets 5 ± 3 km thick. After cessation of UHP recrystallization, tectonic slices ascended largely because of buoyancy to shallow depths along stress guides provided by the subduction zones themselves. Collisional sheets that retain UHP relics (micro-inclusions enclosed in strong, impermeable, unreactive mineralogic host grains) lost heat by conduction across both upper, normal-fault and lower, reverse-fault contacts. These sheets rose to mid-crustal levels rapidly at exhumation rates approaching 10 mm/yr. Backreaction attending decompression in all cases was nearly complete; where UHP relics survive, retrogression evidently was limited by the coarse grain size and relative impermeability of the rocks, as well as by declining temperature and lack of aqueous fluids.     

Exhumation of oceanic eclogites: thermodynamic constraints on pressure,temperature, bulk composition and density

The exhumation mechanism of high‐pressure (HP) and ultrahigh‐pressure (UHP) eclogites formed by the subduction of oceanic crust (hereafter referred to as oceanic eclogites) is one of the primary uncertainties associated with the subduction factory. The phase relations and densities of eclogites with MORB compositions are modelled using thermodynamic calculations over a P–T range of 1–4 GPa and 400–800 °C, respectively, in the NCKFMASHTO (Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O–TiO₂–Fe₂O₃) system. Our modelling suggests that the mineral assemblages, mineral proportions and density of oceanic crust subducted along a cold P–T path are quite different from those of crust subducted along a warm P–T path, and that the density of oceanic eclogites is largely controlled by the stability of low‐density hydrous minerals, such as lawsonite, chlorite, glaucophane and talc. Along a cold subduction P–T path with a geotherm of ~6 °C km^?1, lawsonite is always present at 1.1 to >4.0 GPa, and chlorite, glaucophane and talc can be stable at pressures of up to 2.3, 2.6 and 3.6 GPa respectively. Along such a P–T path, the density of subducted oceanic crust is always lower than that of the surrounding mantle at depths shallower than 110–120 km (< 3.3–3.6 GPa). However, along a warm subduction P–T path with a geotherm of ~10 °C km^?1, the P–T path is outside the stability field of lawsonite, and the hydrous minerals of chlorite, epidote and amphibole break down completely into dry dense minerals at relatively lower pressures of 1.5, 1.85 and 1.9 GPa respectively. Along such a warm subduction P–T path, the subducted oceanic crust becomes denser than the surrounding mantle at depths >60 km (>1.8 GPa). Oceanic eclogites with high H₂O content, oxygen fugacity, bulk‐rock X_Mg [ = MgO/(MgO + FeO)], X_Al [ = Al₂O₃/(Al₂O₃ + MgO + FeO)] and low X_Ca [ = CaO/(CaO + MgO + FeO + Na₂O)] are likely suitable for exhumation, which is consistent with the bulk‐rock compositions of the natural oceanic eclogites on the Earth's surface. On the basis of natural observations and our calculations, it is suggested that beyond depths around 110–120 km oceanic eclogites are not light enough and/or there are no blueschists to compensate the negative buoyancy of the oceanic crust, therefore explaining the lack of oceanic eclogites returned from ultradeep mantle (>120 km) to the Earth's surface. The exhumed light–cold–hydrous oceanic eclogites may have decoupled from the top part of the sinking slab at shallow depths in the forearc region and are exhumed inside the serpentinized subduction channel, whereas the dense–hot–dry eclogites may be retained in the sinking slab and recycled into deeper mantle.     

Boron isotopic composition of subduction-zone metamorphic rocks

Many arc lavas contain material derived from subducted oceanic crust and sediments, but it remains unresolved whether this distinctive geochemical signature is transferred from the subducting slab by aqueous fluids, silicate melts, or both. Boron isotopic measurements have the potential to distinguish between slab transfer mechanisms because ¹¹B fractionates preferentially into aqueous fluids whereas little fractionation may occur during partial melting. Previous studies have shown that δ¹¹B values of island arc lavas (−6 to +7) overlap the range of δ¹¹B values for altered oceanic crust (−5 to +25) and pelagic sediments and turbidites (−7 to +11). Secondary ion mass spectrometry (SIMS) analyses of minerals in subduction-zone metamorphic rocks yield δ¹¹B=−11 to −3 suggesting that slab dehydration reactions significantly lower the δ¹¹B values of subducted oceanic crust and sediments. In order to explain the higher δ¹¹B values reported for arc lavas as compared to subduction-zone metamorphic rocks, the B-bearing component derived from the metamorphosed slab must be enriched in ¹¹B relative to the slab, favoring an aqueous fluid as the slab transfer mechanism.     

Protoliths and phase petrology of whiteschists

Whiteschists appear in numerous high- and ultrahigh-pressure rock suites and are characterized by the mineral assemblage kyanite + talc (+-quartz or coesite). We demonstrate that whiteschist mineral assemblages are well stable up to pressures of more than 4 GPa but may already form at pressures of 0.5 GPa. The formation of whiteschists largely depends on the composition of the protolith, which requires elevated contents of Al and Mg as well as low Fe, Ca, and Na contents, as otherwise chloritoid, amphibole, feldspar, or omphacite are formed instead of kyanite or talc. Furthermore, the stability field of the whiteschist mineral assemblage strongly depends on XCO₂ and fO₂: already at low values of XCO₂, CO₂ binds Mg to carbonates strongly reducing the whiteschist stability field, whereas high fO₂ enlarges the stability field and stabilizes yoderite. Thus, the scarcity of whiteschist is not necessarily due to unusual P–T conditions, but to the restricted range of suitable protolith compositions and the spatial distribution of these protoliths: (1) continental sedimentary rocks and (2) hydrothermally and metasomatically altered felsic to mafic rocks. The continental sedimentary rocks that may produce whiteschist mineral assemblages typically have been deposited under arid climatic conditions in closed evaporitic basins and may be restricted to relatively low latitudes. These rocks often contain large amounts of the clay minerals palygorskite and sepiolite. Marine sediments generally do not yield whiteschist mineral assemblages as marine shales commonly have too high iron contents. Sabkha deposits may have too high CO₂ contents. Protoliths of appropriate geochemical composition occur in and on continental crust. Therefore, whiteschist assemblages typically are only found in settings of continental collision or where continental fragments were involved in subduction. Our calculations demonstrate that whiteschists can form by closed-system metamorphism, which implies that the chemical and isotopic composition of these rocks provide constraints on the development of the protoliths.     

Multi-stage magma ascent beneath the Canary Islands: evidence from fluid inclusions

Gabbroic and ultramafic xenoliths and olivine and clinopyroxene phenocrysts in basaltic rocks from Gran Canaria, La Palma, El Hierro, Lanzarote and La Gomera (Canary Islands) contain abundant CO₂-dominated fluid inclusions. Inclusion densities are strikingly similar on a regional scale. Histogram maxima correspond to one or more of the following pressures: (1) minimum 0.55 to 1.0 GPa (within the upper mantle); (2) between 0.2 and 0.4 GPa (the Moho or the lower crust); (3) at about 0.1 GPa (upper crust). Fluid inclusions in several rocks show a bimodal density distribution, the lower-density maximum comprising both texturally early and late inclusions. This is taken as evidence for an incomplete resetting of inclusion densities, and simultaneous formation of young inclusions, at well-defined magma stagnation levels. For Gran Canaria, pressure estimates for early inclusions in harzburgite and dunite xenoliths and olivine phenocrysts in the host basanites overlap at 0.9 to 1.0 GPa, indicating that such magma reservoir depths coincide with levels of xenolith entrainment into the magmas. Magma chamber pressures within the mantle, inferred to represent levels of mantle xenolith entrainment, are 0.65–0.95 GPa for El Hierro, 0.60–0.68 GPa for La Palma, and 0.55–0.75 GPa for Lanzarote. The highest-density fluid inclusions in many Canary Island mantle xenoliths have probably survived in-situ near-isobaric heating at the depth of xenolith entrainment. Inclusion data from all islands indicate ponding of basaltic magmas at Moho or lower crustal depths, and possibly at an additional higher level, strongly suggestive of two main crustal accumulation levels beneath each island. We emphasize that repeated magmatic underplating of primitive magmas, and therefore intrusive accretion, are important growth mechanisms for the Canary Islands, and by analogy, for other ocean islands. Comparable fluid inclusion data from primitive rocks in other tectonic settings, including Iceland, Etna and continental rift systems (Hungary, South Norway), indicate that magma accumulation close to Moho depths shortly before eruption is not, however, restricted to oceanic intraplate volcanoes. Lower crustal ponding and crystallization prior to eruption may be the rule rather than the exception, independent of the tectonic setting. Received: 30 May 1997 / Accepted: 6 February 1998     

Petrology of an Arc-Oceanic Crust Contact Zone in the Laohushan Back-arc Basin, the Eastern Section of the North Qilian Mountains, NW China

A contact zone sandwiched between an arc and an oceanic crust was discovered in the Laohushan area in the present study. It consists of a series of north-dipping imbricated thrust sheets and is exposed on the surface as a narrow arcuate belt, which extends for about 30 km in an E-W direction and measures about 1-3 km wide. Lithologically, it can be divided into four subzones. Subzone 1 consists of meta-andesite and metasandstone; subzone 2, psammitic schists; subzone 3, psammitic and pelitic schists, quartz diorite and hornfelses; and subzone 4, metagabbro, epidote amphibolite and pelitic schists. The metamorphism has the following grading sequence: low greenschist facies in subzone 1 → high greenschist facies in subzone 2 →low amphibolite facies in subzone 3→ epidote amphibolite facies in subzone 4. Petrographic and geochemical evidence shows that rocks in subzones 1, 2 and 3 are arc rocks, whereas those of subzone 4 are oceanic crustal rocks. The metamorphic mineral assemblages and especially miner     

实验岩石学对埃达克岩成因的限定——兼论中国东部富钾高Sr/Y比值花岗岩类

大量变玄武岩脱水熔融实验表明,制约埃达克岩形成的主要因素是源岩、水和地壳热结构(p-T轨迹)。变玄武岩低到中等程度(10％～40％)的部分熔融过程中,含水矿物(主要是角闪石)脱水反应产生埃达克岩熔体,残余相组合为石榴石+单斜辉石±斜方辉石±角闪石(没有斜长石)。在俯冲带,当压力为1.6～2.2 GPa(约70～90 km)和温度为800～1150℃时,具有高的剪切热速率和非常年轻的(<25 Ma)、热的俯冲大洋岩石圈就会发生脱水熔融形成埃达克熔体。在增厚地壳内,具有高的热状态的底侵玄武质下地壳在压力≥0.8 GPa(>35 km)和温度介于800～1100℃之间发生部分熔融形成埃达克质熔体。然而,中国东部晚中生代富钾高Sr/Y比值花岗岩类,可能形成于加厚地壳开始减薄及地壳从挤压向拉张伸展转换的环境下,所对应的岩浆,与下地壳底侵的碱性玄武岩和/或拉斑玄武岩在压力1.0～1.5 GPa和温度850～1080℃之间发生部分熔融有关,熔融的残余相为辉石岩类,岩浆在上升侵位过程中还受到了地壳AFC的影响。中国东部中生代岩石圈从加厚转变为减薄的过程,就可能与玄武质岩浆的底侵作用及随后含石榴石辉石岩类残余体的拆沉作用有关。     

Aluminous enstatites of lunar meteorites and deep-seated lunar rocks

Fragments of aluminous enstatite from lunar meteorites of highland origin were investigated. It was found that such fragments usually occur in impact breccias of troctolitic composition. The aluminous enstatite contains up to 12 wt % Al₂O₃ and shows low CaO (<1 wt %) and almost constant high Mg/(Mg + Fe) ratio (89.5 ± 1.4 at %) identical to that of the Earth’s mantle. With respect to these parameters, the aluminous enstatites are distinctly different from common orthopyroxene of lunar rocks. The aluminous enstatite associates with spinel (pleonaste), olivine, anorthite (clinopyroxene was never found), and accessory minerals: rutile, Ti-Zr oxides, troilite, and Fe-Ni metal. The same assemblage was described in rare fragments of spinel cataclasites from the samples of the Apollo missions. Thermobarometry and the analysis of phase equilibria showed that the rocks hosting aluminous enstatite are of deep origin and occurred at depths from 25 km to 130–200 km at T from 800 to 1300°C, i.e., at least in the lower crust and, possibly, in the upper mantle of the Moon. These rocks could form individual plutons or dominate the composition of the lower crust. The most probable source of aluminous enstatite is troctolitic magnesian rocks and, especially, spinel troctolites with low Ca/Al and Ca/Si ratios. The decompression of such rocks must produce cordierite-bearing assemblages. The almost complete absence of such assemblages in the surficial rocks of lunar highlands implies that vertical tectonic movements were practically absent in the lunar crust. The transport of deep-seated materials to the lunar surface was probably related to impact events during the intense meteorite bombardments >3.9 Ga ago.     

Garnet–chloritoid–kyanite assemblages: eclogite facies indicators of subduction constraints in orogenic belts

The assemblage garnet–chloritoid–kyanite is shown to be quite common in high‐pressure eclogite facies metapelites from orogenic belts around the world, and occurs over a narrowly restricted range of temperature ～550–600 °C, between 20 and 25 kbar. This assemblage is favoured particularly by large Al₂O₃:K₂O ratios allowing the development of kyanite in addition to garnet and chloritoid. Additionally, ferric iron and manganese also help stabilize chloritoid in this assemblage. Pseudosections for several bulk compositions illustrate these high‐pressure assemblages, and a new thermodynamic model for white mica to include calcium and ferric iron was required to complete the calculations. It is extraordinary that so many orogenic eclogite facies rocks, both mafic eclogites sensu stricto as well as metapelites with the above assemblage, all yield temperatures within the range of 520–600 °C and peak pressures ～23±3 kbar. Subduction of oceanic crust and its entrained associated sedimentary material must involve the top of the slab, where mafic and pelitic rocks may easily coexist, passing through these P–T conditions, such that rocks, if they proceed to further depths, are generally not returned to the surface. This, together with the tightly constrained range in peak temperatures which such eclogites experience, suggests thermal weakening being a major control on the depths at which crustal material is decoupled from the downgoing slab.     

Ar degassing and the origin of the sialic crust

The degassing of radiogenic Ar⁴⁰ is defined as coherent if only the Ar⁴⁰ associated with parent K is degassed as K is transferred from the mantle to crust. Coherency predicts, for a 4.55 b.y. Earth, a sialic crust with 2.50 per cent K, using only the Ar content of the atmosphere and present crust (from a Hurley and Rand, 1969, age distribution). This is a maximum limit to K content of the sialic crust if the age of the Earth is no younger than 4.55 b.y. A K content of the sialic crust of 1.9 per cent (Holland and Lambert, 1972) implies an efficiency (E) less than 100 per cent for K transfer from oceanic basalt to sialic crust in subduction zones and/or some non-coherent (preferential) degassing of Ar from the mantle.K, Ar coherence for mantle differentiation to crust is supported however, by the agreement of the predicted oceanic He flux and radiogenic He-Ar ratios of volcanic gases with the observed limits if the best estimate of K, U, Th influx rates at oceanic ridges is used.Assuming K, Ar coherence, various sea-floor spreading rates as functions of time, and limiting K contents of the sialic crust, computed models give E and the portion of the sialic crust derived from melting oceanic basalt in subduction zones. Except for models with very high spreading rates in the Precambrian, they also predict that a significant part of the sialic crust was derived from vertical differentiation of the mantle, presumably early in Earth history. The results are in accord with Armstrong's model of an early sialic crust that is recycled to give a Hurley-type age pattern with the proviso that the ‘vertical’ sial K_υis formed early in Earth history for models with a high K_υcomponent.The coherent K, Ar models with preferred estimates of input parameters are also consistent with a limited mixing model (only old and new sial are equilibrated) for Sr isotopic evolution and the probable average $\text{Sr}{}^{87}\text{Sr}{}^{86}$ ratio now of the sialic crust.     

Subduction and exhumation mechanisms of ultra‐high and high‐pressure oceanic and continental crust at Makbal (Tianshan,Kazakhstan and Kyrgyzstan)

The Makbal Complex in the northern Tianshan of Kazakhstan and Kyrgyzstan consists of metasedimentary rocks, which host high‐P (HP) mafic blocks and ultra‐HP Grt‐Cld‐Tlc schists (UHP as indicated by coesite relicts in garnet). Whole rock major and trace element signatures of the Grt‐Cld‐Tlc schist suggest a metasomatized protolith from either hydrothermally altered oceanic crust in a back‐arc basin or arc‐related volcaniclastics. Peak metamorphic conditions of the Grt‐Cld‐Tlc schist reached ~580 °C and 2.85 GPa corresponding to a maximum burial depth of ~95 km. A Sm‐Nd garnet age of 475 ± 4 Ma is interpreted as an average growth age of garnet during prograde‐to‐peak metamorphism; the low initial εΝd value of ?11 indicates a protolith with an ancient crustal component. The petrological evidence for deep subduction of oceanic crust poses questions with respect to an effective exhumation mechanism. Field relationships and the metamorphic evolution of other HP mafic oceanic rocks embedded in continentally derived metasedimentary rocks at the central Makbal Complex suggest that fragments of oceanic crust and clastic sedimentary rocks were exhumed from different depths in a subduction channel during ongoing subduction and are now exposed as a tectonic mélange. Furthermore, channel flow cannot only explain a tectonic mélange consisting of various rock types with different subduction histories as present at the central Makbal Complex, but also the presence of a structural ‘dome’ with UHP rocks in the core (central Makbal) surrounded by lower pressure nappes (including mafic dykes in continental crust) and voluminous metasedimentary rocks, mainly derived from the accretionary wedge.     

中祁连地块北缘退变榴辉岩的发现及其地质意义

1∶5万区域地质调查首次在中祁连地块北缘发现的退变榴辉岩,呈构造岩块分布于大羊陇一带的变质基底中。岩相学和矿物学研究显示,石榴石的矿物包体和化学成分具有进变质环带的特征,属于C类榴辉岩。石榴石核部成分以及残留于核部的黑云母、斜长石等矿物包体代表了进变质阶段(M1)矿物组合,计算得到其温压条件为568～580 ℃和0.80～0.82 GPa。大致估算得峰期榴辉岩相阶段(M2)温压条件为(669±5) ℃和(2.1±0.2) GPa。石榴石“白眼圈”结构指示了等温减压退变质作用,利用局部的平衡矿物获得高角闪岩相退变质阶段(M3)的温压条件为681～705 ℃和0.68～0.71 GPa。进一步的退变质作用发生在低角闪岩相条件下,以基质中出现粗粒的角闪石和斜长石为特征,估算得到这一阶段(M4)温压分别为500～545 ℃和0.38～0.43 GPa。上述变质过程形成一个顺时针的p-T演化轨迹,暗示板片经历过快速俯冲和折返。榴辉岩的锆石CL图像显示锆石大部分发光度低,为无分带、弱分带或海绵状分带,边部发育宽约5 μm的强阴极发光带,主体表现为变质增生锆石的特征。LA-ICP-MS锆石U-Pb定年获得峰期榴辉岩相变质的上限年龄为(485±22) Ma。根据岩石地球化学特征和构造环境判别,大羊陇榴辉岩的原岩为MORB,推测属于北祁连洋壳的组成部分。结合中祁连地块北缘广泛发育弧岩浆岩,确定了晚寒武世-中奥陶世北祁连洋壳存在向南的俯冲作用,其俯冲极性为南北双向俯冲。     

New data on relationship between earth's crust and upper mantle

New data on velocities of elastic waves in rocks and minerals under pressures, anisotropism of elastic properties of monocrysts, oceanic volcanism, deep seismic profiles, isotopic composition of strontium and distribution-abundance of Sr⁸⁷ in ancient and in young rocks, and others, tend to show that the "M" discontinuity is but an expression of the state of compaction of the rocks, devoid of any petrographic or geochemical connotations, that the sialitic shell of the Earth, with its heterogeneities, extends to depths exceeding 100 km (i.e. deeper than the "M"), and that the two types of the crust, "oceanic" and "continental," created by geophysicists, are actually one and the same type. Tentatively drawn analogies between the terrestrial and the lunar crust, on the assumption of a lunar origin of tektites, and the.crust of Mars, with regard to densities and planetary size relationships, are used as illustrations of the re-evaluated ratios between the crust and the upper mantle of the earth. — IGR Staff.     

Metamorphic consequences of secular changes in oceanic crust composition and implications for uniformitarianism in the geological record

Cooling of the Earth's mantle since the Meso-Archean is predicted by thermal and petrological models to have induced a secular change in the composition of primary mantle-derived magmas-and thus bulk oceanic crust; in particular, suggesting a decrease in maficity over time. This hypothesis underpins several recent studies that have addressed key geological questions concerning evolving plate tectonic styles, the rates and timing of continental crust formation, comparative planetology, and the emergence of complex life on Earth. Major, minor, and trace element geochemical analyses of(meta)mafic rocks preserved in the geological record allows exploration of this theory, although no consensus currently exists about the magnitude of this change and what compositions-if anything-constitute representative examples of Paleo-, Meso-, or Neo-Archean primitive oceanic crust. In this work, we review the current state of understanding of this issue, and use phase equilibria to examine the different mineral assemblages and rock types that would form during metamorphism of basalt of varying maficity in subduction zone environments. The presence(or absence) of such metamorphic products in the geological record is often used as evidence for(or against) the operation of modern-day subductiondriven plate tectonics on Earth at particular time periods; however, the control that secular changes in composition have on the stability of mineral assemblages diagnostic of subduction-zone metamorphism weakens such uniformitarianistic approaches. Geodynamic interpretations of the Archean metamorphic rock record must therefore employ a different set of petrological criteria for determining tectonothermal histories than those applied to Proterozoic or Phanerozoic equivalents.     

Fluids, tectonics and crustal deformation

In the plate tectonic process, lithosphere creation at ocean ridges and its cooling leads to volatile fixation in the oceanic crust. The outer 10 km or so of all crust contains abundant water in pores and fractures and variable amounts of volatiles in minerals. When surface rocks are buried by tectonic processes, fluids must be released and modify the mechanical properties. In the subduction process hydrated oceanic crust may be decoupled from the remaining oceanic lithosphere. At depth rising aqueous fluids or melts lead to a complex series of mass-energy transfer processes which may decouple continental crust near the Moho. Continental crust if subducted, may also be decoupled from its lithosphere by degassing. Fluid release processes which create gas-solid mixtures beneath impermeable cover create low-strength systems subject to facile deformation, hydraulic fracture processes and diapiric phenomena.     

Exhumation of oceanic blueschists and eclogites in subduction zones: Timing and mechanisms

High-pressure low-temperature (HP–LT) metamorphic rocks provide invaluable constraints on the evolution of convergent zones. Based on a worldwide compilation of key information pertaining to fossil subduction zones (shape of exhumation P–T–t paths, exhumation velocities, timing of exhumation with respect to the convergence process, convergence velocities, volume of exhumed rocks,…), this contribution reappraises the burial and exhumation of oceanic blueschists and eclogites, which have received much less attention than continental ones during the last two decades.Whereas the buoyancy-driven exhumation of continental rocks proceeds at relatively fast rates at mantle depths (≥ cm/yr), oceanic exhumation velocities for HP–LT oceanic rocks, whether sedimentary or crustal, are usually on the order of the mm/yr. For the sediments, characterized by the continuity of the P–T conditions and the importance of accretionary processes, the driving exhumation mechanisms are underthrusting, detachment faulting and erosion. In contrast, blueschist and eclogite mafic bodies are systematically associated with serpentinites and/or a mechanically weak matrix and crop out in an internal position in the orogen.Oceanic crust rarely records P conditions > 2.0–2.3 GPa, which suggests the existence of maximum depths for the sampling of slab-derived oceanic crust. On the basis of natural observations and calculations of the net buoyancy of the oceanic crust, we conclude that beyond depths around 70 km there are either not enough serpentinites and/or they are not light enough to compensate the negative buoyancy of the crust.Most importantly, this survey demonstrates that short-lived (< ～ 15 My), discontinuous exhumation is the rule for the oceanic crust and associated mantle rocks: exhumation takes place either early (group 1: Franciscan, Chile), late (group 2: New Caledonia, W. Alps) or incidentally (group 3: SE Zagros, Himalayas, Andes, N. Cuba) during the subduction history. This discontinuous exhumation is likely permitted by the specific thermal regime following the onset of a young, warm subduction (group 1), by continental subduction (group 2) or by a major, geodynamic modification of convergence across the subduction zone (group 3; change of kinematics, subduction of asperities, etc).Understanding what controls this short-lived exhumation and the detachment and migration of oceanic crustal slices along the subduction channel will provide useful insights into the interplate mechanical coupling in subduction zones.     

An interpretation of the 1883 cataclysmic eruption of Krakatau from geochemical studies on the partial melting of granite

Pumice flow from the 1883 Krakatau eruption significantly differs in both mineral and chemical compositions from any other volcanic rocks or ejecta of the Krakatau group, which belong to the tholeiitic series. Lithic fragments of granitic Rock, discovered in the pumice flow, are similar to West Malayan granitic rocks. No other granitic rock occurs throughout the Krakatau group, therefore, we consider that the granitic fragments came from the underlying complex at depths, where they were captured as foreign materials by the magma.It is possible that sialic crustal materials plunged into depths along a peculiar tectonic structure located at the Sunda Strait, which appears to be a sheared portion caused by deformation of the Sunda arc due to differential movement between the Indo-Australian oceanic plate and the Eurasian continental crust. The crustal materials were partially melted and produced a magma of granitic composition. The magma was mixed with or assimilated by an ascending basaltic magma originating probably from the upper mantle. This resulted in a dacitic magma distinctly dominant in silica, alkalis and volatile components, and the 1883 Krakatau eruption, characterized by the pumice flow of dacitic composition, took place.