New, high-precision P–T estimates for Oman blueschists: implications for obduction, nappe stacking and exhumation processes

Oman blueschists and eclogites lie below the obduction nappe of the Semail ophiolite in one of the key areas on Earth for the study of plate convergence. Here new metamorphic and tectonic constraints are provided for the central, yet poorly constrained Hulw unit, sandwiched between the low‐grade units (～10 kbar, <300 °C) and the As Sifah eclogites (P_max ～ 23 kbar; T_max ～ 600 °C). TWEEQU multi‐equilibrium thermobarometry, using both compositional mapping and spot analyses, and Raman spectroscopy of carbonaceous material yield a high‐precision P–T path for the Hulw and As Sheikh units and reveal that they shared a common P–T history in four stages: (i) a pressure decrease from 10–12 kbar, 250–300 °C to 7–9 kbar, 300–350 °C; (ii) almost isobaric heating at ～8–10 kbar from 300–350 °C to 450–500 °C; (iii) a pressure decrease at moderate temperatures (～450–500 °C); and (iv) isobaric cooling at ～5–6 kbar from 450–500 to 300 °C. No significant pressure or temperature gap is observed across the upper plate–lower plate discontinuity to the north and west of the Hulw unit. The combination of tectonic and P–T data constrains the stacking chronology of the three main metamorphic units comprising the Saih Hatat window (i.e. the Ruwi‐Quryat, the Hulw‐As Sheikh and the Diqdah‐As Sifah units). These results strengthen the view that the tectonic and metamorphic data are conveniently accounted for by a simple, N‐vergent continental subduction of the passive Arabian margin below the obduction nappe along a cold P–T gradient.     

High pressure metamorphism in the nappes of the Valle dell'Orco traverse (Western Alps collisional belt) *

Abstract A detailed field and petrological study of rocks from nappes cut by the Valle dell'Orco (Italian Western Alps), in particular the Sesia–Lanzo composite unit, has revealed geological and metamorphic histories which started in pre-alpine times and lasted up to the alpine subduction-collisional processes. During these processes the nappes sustained an early high P–low T stage and a later low P greenschist facies stage, but followed partly distinctive P–T–time trajectories. This paper discusses the kinematic evolution and the thermal history of the alpine belt from the early subduction/underthrust to the later exhumation stage. The metamorphic crystallization is often governed by incomplete and/or local equilibrium, and the pervasive syn-metamorphic deformation and the composition of the syn-metamorphic fluid phase (if present) have exerted an effective local control on reaction kinetics.     

Domal structures and high-grade metamorphism in the Higher Himalayan Crystalline, Zanskar Region, north-west Himalaya, India

ABSTRACT In the main Himalayan range in the Ladakh-Zanskar area, domal structures have been observed at structurally deeper levels in the tectonic unit of the Higher Himalayan Crystalline. Their formation occurred during a second, temperature-dominated phase (M₂) of high-grade regional metamorphism, characterized by the semipelitic paragenesis of sillimanite-K-feldspar and incipient anatexis. The doming event reveals a local system of synmetamorphic uplift superimposed on a regional system of northeast-southwest trending compression. In the main Himalayan range the development of the dominant S₂ foliation is related to deformation during the doming phase, which started early in the M₂ event. The deformation propagated continuously north-east and south-west with time. In the north-east, on the northern slopes of the main Himalayan range, this deformation is expressed by extensional shear movements of the upper tectonic levels finally leading to the late- to postmetamorphic normal fault system of the Zanskar shear zone. Towards the south-west, deformation is expressed by compressional movements, e.g. at the Main Central Thrust (MCT) in the Kishtwar window area. The observed compression and extension is inferred to relate to an increased uplift of the domal bulges of the tectonic Kishtwar window and of the whole main Himalayan range.     

Alpine metamorphism of Hercynian hornblende granodiorite beneath the blueschist facies schistes lustrés nappe of NE Corsica

Abstract The Hercynian granitic basement which forms the Tenda Massif in NE Corsica represents part of the leading edge of the European Plate during middle-to-late Cretaceous (Eoalpine) high P metamorphism. The metamorphism of this basement, induced by the overthrusting of a blueschist facies (schistes lustrés) nappe, was confined to a major ductile shear zone (c. 1000m thick) within which deformation increases upwards towards the overlying nappe. Metamorphism within the basement mostly records lower blueschist facies conditions (crossite + epidote) except near the base of the shear zone where the greenschist facies assemblage albite + actinolitic amphibole has developed instead of crossite. Study of the primary mafic phase breakdown reactions within hornblende granodiorite reveals the following metamorphic zonation. Zone 1: biotite to chlorite. Towards zone 2: biotite to phengite. Zone 2: Hornblende to actinolitic Ca-amphibole + albite + sphene, and biotite to actinolitic Ca-amphibole + albite + phengite + Ti-ore + epidote. Zone 3: Hornblende to crossite + low Ti-biotite + phengite + sphene, and biotite to crossite + low Ti-biotite + phengite + Ti-ore + sphene ± epidote. P-T conditions at the base of the shear zone are estimated to have been 390-490°C at 600-900 M Pa (6-9kbar) and the Corsican basement is therefore deduced to have been buried to 20-30 km during metamorphism. This relatively shallow metamorphism contrasts with some other areas in the Western Alps where the Eoalpine event apparently buried the European continental crust to depths of 80 km or more. As there is no evidence for a long history of blueschist facies metamorphism prior to the involvement of the European continent, it is deduced that the Eoalpine blueschists were produced during the collision of the Insubric plate with Europe, rather than during Tethyan intraoceanic subduction. Coherent blueschist terrains such as the schistes lustres probably record buovant feature collision and obduction tectonics rather than any preceding oceanic subduction.     

Shock metamorphism on the moon

Shock metamorphism of the lunar samples is discussed. All types of lunar glasses formed by various-size collision-type impact are found as impact glass, ropy glass and agglutinates. The agglutinates bonded by crystal and glassy materials contain hydrogen and helium from the solar wind components. Lunar shocked minerals of plagioclase and silica show anomalous compositions and densities. There are typical two formation processes on planetary materials formed by shock events; that is (1) shocked quartz formed by silica-rich target rocks (esp. on evolved planets of the Earth and Mars), and (2) shocked silica with minor Al contents formed from plagioclase-rich primordial crusts of the Moon. The both shocked silica grows to coarse-grain normal crystals after high-temperature metamorphism which cannot distinguish the original main formation event of impact process.     

Timing of high-grade metamorphism: Early Palaeozoic U–Pb formation ages of titanite indicate long-standing high-T conditions at the western margin of Gondwana (Argentina, 26–29°S)

Abstract Concordant U–Pb ages of c. 530–510 Ma and c. 470–420 Ma on titanite from calcsilicate, orthogneiss and amphibolite rocks constrain the age of high‐T metamorphism in the Early Palaeozoic mobile belt at the western margin of Proterozoic Gondwana (Argentina, 26–29°S). The U–Pb ages document the time of titanite formation at high‐T conditions according to the stable mineral paragenesis and occurrence of titanite in the metamorphic fabric. The presence of migmatite at all sample sites indicates temperatures were > c. 650 °C. Titanite formed at similar metamorphic conditions at different times on the regional and on the outcrop scale. The titanite crystals preserved their U–Pb isotopic signatures and chemical composition under ongoing upper amphibolite to granulite facies temperatures. Different thermal peaks or deformations are only detected by the different U–Pb ages and not by changes in the mineral paragenesis or metamorphic fabric of the samples. The range of U–Pb ages, e.g. in the Ordovician and Silurian (c. 470, 460, 440, 430, 420 Ma), is interpreted as the effect polyphase deformation with deformation‐enhanced recrystallization of titanite and/or different thermal peaks during a long‐standing, geographically fixed, high‐T regime in the mid‐crust of a continental magmatic arc. A clear correlation of the different ages with distinct tectonic events, e.g. collision of terranes, is not possible based on the present knowledge of the region.     

Stable isotope constraints on the Al2SiO5'triple-point' rocks from the Proterozoic Priest pluton contact aureole, New Mexico, USA

The Priest pluton contact aureole in the Manzano Mountains, central New Mexico preserves evidence for upper amphibolite contact metamorphism and localized retrograde hydrothermal alteration associated with intrusion of the 1.42 Ga Priest pluton. Quartz–garnet and quartz–sillimanite oxygen isotope fractionations in pelitic schist document an increase in the temperatures of metamorphism from 540 °C, at a distance of 1 km from the pluton, to 690 °C at the contact with the pluton. Comparison of calculated temperature estimates with one‐dimensional thermal modelling suggests that background temperatures between 300 and 350 °C existed at the time of intrusion of the Priest pluton. Fibrolite is found within 300 m of the Priest pluton in pelitic and aluminous schist metamorphosed at temperatures >580 °C. Coexisting fibrolite and garnet in pelitic schist are in oxygen isotope equilibrium, suggesting these minerals were stable reaction products during peak metamorphism. The fibrolite‐in isograd is coincident with the staurolite‐out isograd in pelitic schist, and K‐feldspar is not observed with the first occurrence of fibrolite. This suggests that the breakdown of staurolite and not the second sillimanite reaction controls fibrolite growth in staurolite‐bearing pelitic schist. Muscovite‐rich aluminous schist locally preserves the Al₂SiO₅ polymorph triple‐point assemblage – kyanite, andalusite and fibrolite. Andalusite and fibrolite, but not kyanite, are in isotopic equilibrium in the aluminous schist. Co‐nucleation of fibrolite and andalusite at 580 °C in the presence of muscovite and absence of K‐feldspar suggests that univariant growth of andalusite and fibrolite occurred. Kyanite growth occurred during an earlier regional metamorphic event at a temperature nearly 80 °C lower than andalusite and fibrolite growth. Quartz–muscovite fractionations in hydrothermally altered pelitic schist and quartzite are small or negative, suggesting that late isotopic exchange between externally derived fluids and muscovite, but not quartz, occurred after peak contact metamorphism and that hydrothermal alteration in pelitic schist and quartzite occurred below the closure temperature of oxygen self diffusion in quartz (<500 °C).     

贺兰山群变质杂岩中的变余动力变质岩

贺兰山群变质杂岩形成于晚太古代,遭受过多期变质和变形的改造,其中发育了不同期次和层次的动力变质岩。变余韧性剪切带内发育了条带状片麻岩和变粒岩,相伴有变余鞘褶皱和构造透镜体,变余韧性剪切带的变形发生在较低的温度、压力条件下,地壳加厚作用的初期。变余糜棱岩貌似条带状片麻岩,但在青石中保留了早期石英拔丝构造和加粗的动态重结晶长石集合体。糜棱岩化作用的温度在550℃以上,变形后沿面理形成了饼状或椭圆状青石,变晶时温度近600℃,压力近300MPa。     

Pervasive granitoid magma transfer through the lower–middle crust during non-coaxial compressional deformation

Granitic magmas migrated through Early Proterozoic middle–lower crust at Mt Hay, central Australia, via a diverse network of narrow structurally controlled channelways, during a period of progressive W–SW-directed thrusting (D1a–D1d). They utilized existing folds, boudins and shear zones, or created new channels by magmatic fracture either parallel to layering or, rarely, in irregular arrays. The magmas rose obliquely, parallel to the plunging (50–60°) regional elongation direction, which was defined by coaxial folds, boudin necks and a strong mineral-elongation lineation. Megacrystic charnockitic magmas migrated through metre-scale conduits during D1a–D1b, but leucosomes were generally restricted to smaller (centimetre-scale) structures that existed throughout the entire deformation history. Thus, D1a/D1b leucosomes were potential feeders of in situ partial melts to the adjacent larger conduits of charnockite magma, thereby providing a pervasive interconnected network that allowed efficient migration of all magma types during the early stages of thrusting. The upper–middle crust of the Anmatjira–Reynolds Range area contains abundant megacrystic granitoid sheets that are of similar age and geochemistry to those at Mt Hay. They are considered to have formed as syntectonic intrusions emplaced during W–SW-directed thrusting, as at Mt Hay, suggesting that granitic magmas formed near the base of the continental crust passed through the mid-lower crustal level (25–30 km) exposed at Mt Hay and accumulated, in batholithic proportions, at shallower crustal levels (12–20 km) such as the Anmatjira–Reynolds Range area. The observations imply that granitoid magmas in the deep crust are capable of pervasive migration through the crust during major compressive, noncoaxial shear deformation. Localization of magmas by sequentially developed, narrow, compressive structures suggests that dilatancy followed successive foliation-forming events, a situation that can occur during steady-state deformation if the effective confining pressures are low, which would be a result of high and possibly variable rates of magma influx. The inferred rapid melt segregation and migration during deformation suggest that large chambers do not form until magma reaches neutral buoyancy in the middle to upper continental crust.     

Metamorphism of the Scourian Complex, NW Scotland

The rocks of the Scourian Complex have been intensively studied, but there is still no consensus as to the conditions of the granulite-facies metamorphism preserved in these rocks. Recent estimates of these conditions fall into two groups, one at 820-920°C and ca. 11 kbar and the second at ca. 1000°C and >12 kbar. Investigation of a variety of rocks shows that the recorded conditions vary with grain-size, with higher-grade conditions recorded by the cores of coarser ( ca. 10 mm) crystals, and lower-grade conditions recorded by the rims of coarser grains and by finer grains. This observation suggests that re-equilibration during recovery of these rocks to the surface has been important which may account for the discrepancy in estimated P-T conditions. Revised estimates of the equilibration conditions of the Scourian Complex of T > 1000°C and P > 8.5 kbar are presented. The conditions suggested for the peak of metamorphism mean that the role of anatexis in the genesis of these rocks must be considered and the nature of the fluid phase thoroughly investigated.