Reconciliation of conflicting tectonic models for Proterozoic rocks of northern New Mexico

Two major Proterozoic tectonic events are documented in the Taos Range of northern New Mexico. Regional structures involving the tectonic interleaving of c.   1.65  Ga granitoids with supracrustal rocks are interpreted to have formed before 1.42  Ga and probably during collisional assembly of island arc crust into new (1.7–1.6  Ga) continental lithosphere. Supracrustal rocks record 650–750  °C, 6–8  kbar metamorphism (M₂); these high temperatures may have been reached during sandwiching between c.   1.65  Ga granitoids. However, the early history has been obscured by renewed tectonism at c.   1.4  Ga that resulted in partial melting, fabric reactivation and new mineral growth at 4  kbar (M₃). Metamorphic temperature variations from uppermost-amphibolite to amphibolite facies rocks may be associated with c.   1.65 and/or 1.4  Ga plutonism, but not to a 1.4  Ga extensional shear zone as previously proposed. Syn- and post-1.4  Ga contraction is suggested by high- and low-temperature microstructures showing top-to-the-south-east thrusting. This work reconciles conflicting models by suggesting that the geometry of the structures was mainly established by c.   1.65  Ga, but that the present fabric also records 1.4  Ga tectonism involving high- T  metamorphism and fabric reactivation.     

Bulk arc strain,crustal thickening,magma emplacement,and mass balances in the Mesozoic Sierra Nevada arc

Quantifying crustal deformation is important for evaluating mass balance, material transfer, and the interplay between tectonism and magmatism in continental arcs. We present a dataset of >650 finite strain analyses compiled from published works and our own studies with associated structural, geochronologic, and geobarometric information in central and southern Sierra Nevada, California, to quantify the arc crust deformation. Our results show that Mesozoic tectonism results in 65% arc-perpendicular bulk crust shortening under a more or less plane strain condition. Mesozoic arc magmatism replaced ∼80% of this actively deforming arc crust with plutons requiring significantly greater crustal thickening. We suggest that by ∼85 Ma, the arc crust thickness was ∼80 km with a 30-km-thick arc root, resulting in a ∼5 km elevation. Most tectonic shortening and magma emplacement must be accommodated by downward displacements of crustal materials into growing crustal roots at the estimated downward transfer rate of 2–13 km/Myr. The downward transfer of crustal materials must occur in active magma channels, or in “escape channels” in between solidified plutons that decrease in size with time and depth resulting in an increase in the intensity of constrictional strain with depth. We argue that both tectonism and magmatism control the thickness of the crust and surface elevation with slight modification by surface erosion. The downward transported crustal materials initially fertilize the MASH zone thus enhancing to the generation of additional magmas. As the crustal root grows it may potentially pinch out and cool the mantle wedge and thus cause reduction of arc magmatism.     

http://www.sciencedirect.com/science/article/pii/S1674987112000679

Quasi-integrity of continental crust between Mid-Archaean and Ediacaran times is demonstrated by conformity of palaeomagnetic poles to near-static positions between～2.7-2.2 Ca,～1.5-1.2 Ga and～0.75-0.6 Ga.Intervening data accord to coherent APW loops turning at "hairpins" focused near a continental-centric location.Although peripheral adjustments occurred during Early Proterozoic （～2.2 Ga） and Grenville（～1.1 Ga） times,the crust retained a low order symmetrical crescent-shaped form constrained to a single global hemisphere until break-up in Ediacaran times.Conformity of palaeomagnetic data to specific Eulerian parameters enables definition of a master Precambrian APW path used to estimate the root mean square velocity(v_RMS) of continental crust between 2.8 and 0.6 Ga.A long interval of little polar movement between～2.7 and 2.2 Ga correlates with global magmatic shutdown between～2.45 and 2.2 Ga,whilst this interval and later slowdown at～0.75-0.6 Ga to velocities of <2 cm/year correlate with episodes of widespread glaciation implying that these prolonged climatic anomalies had an internal origin;the reduced input of volcanically-derived atmospheric greenhouse gases is inferred to have permitted freeze-over conditions with active ice sheets extending into equatorial latitudes as established by low magnetic inclinations in glaciogenic deposits.v_RMS variations through Precambrian times correspond to the distribution of U-Pb ages in orogenic granitoids and detrital zircons and demonstrate that mobility of continental crust has been closely related to crustal tectonism and incrementation.Both periods of near-stillstand were followed by rapid v_RMS recording massive heat release from beneath the continental lid at～2.2 and 0.6 Ga.The first coincided with the Lomagundi-Jatuli isotopic event and led to prolonged orogenesis accompanied by continental flooding and reconfiguration of the crust on the Earth’s surface;the second led to continental break-up and instigated the comprehensive Plate Tectonics that has characterised Phanerozoic times.The Mesoproterozoic interval characterised by anorogenic magmatism correlates with low v_RMS between～1.5 and 1.1 Ga.Insulation of the sub-continental mantle evidently permitted high temperature melting and weakening of the crustal lid to enable buoyant emplacement of large plutons at high crustal levels during this magmatic event unique to Mesoproterozoic and early Neoproterozoic times.     

Geochemistry and evolution of the Wolf River Batholith,a late Precambrian Rapakivi Massif in North Wisconsin,U.S.A.

The Wolf River Batholith is an anorogenic rapakivi massif in central and northeastern Wisconsin with an age of 1.5 Ga. The Batholith has alkaline affinities and consists of biotite granite and biotite-hornblende adamellite with minor occurrences of quartz syenite and older monzonite and anorthosite. The batholith is part of a major Late Precambrian (1.4–1.5 Ga) magmatic event of continental proportions, represented by separate intrusions extending from Labrador to southern California (Silver et al., 1977).The major and trace element composition (Li, Rb, Sr, Ba, and REE) of 40 samples from the anorthosite, monzonite, and rapakivi granite and adamellite plutons precludes a comagmatic (although not cogenetic) model between all three rock units. However, the monzonite may be related to the anorthosite alone by fractional crystallization of plagioclase, orthopyroxene, clinopyroxene, and apatite. Alternatively, the monzonite may be a separate parent melt or a hybrid associated with the granite and adamellite plutons. The high REE content of the monzonite precludes it from being related to the rapakivi granite and adamellite plutons as a source material, a residuum, or a cumulate.A major portion of the Batholith is an undifferentiated intrusive sequence ranging from older rapakivi granite to younger adamellite. The compositions of these plutons suggest a crustal fusion origin at intermediate to lower levels of the crust (25–36 km). The trace element data are consistent with partial fusion of tonalitic to granodioritic source material.During crystallization and emplacement into the upper crust (less than 4 km), 55–70% fractionation of two feldspars, biotite and hornblende from one of the granite plutons produced a small volume of differentiated granitic melt high in Si, Fe/Mg, Rb, Li, and REE (except Eu), and low in Ca, Mg, Al, Ca/Na, Sr, Ba, and K/Rb and with a large negative Eu anomaly. Presumed associated cumulate material ranges from silica-poor quartz monzonite and quartz syenite.The chemical and mineralogical similarity between the Wolf River Batholith and younger magmatic analogs associated in continental break-up (Nigerian younger granites, White Mountain magma series, and the peralkaline volcanics of the Red Sea Region) are suggestive but not conclusive of an extensional tectonic setting. A preliminary tectonic model suggests that the 1.4–1.5 Ga event is in response to thermal doming in an extensional regime leading to continental separation in the western Cordillera (pre-Belt) and extensive crustal fusion with no rifting or separation across the North American Craton.     

Microfabrics of UHP metamorphic granites in the Dora Maira Massif, western Alps – no evidence of deformation at great depth

In the Dora Maira Massif, western Alps, essentially undeformed ultrahigh-pressure (UHP) metamorphic granites (Brossasco granite) are embedded in, and locally grade into, granite gneisses or augengneisses and mylonites. In this study, the quartz microfabrics of the undeformed granites are compared against the augengneisses and mylonites in a representative number of samples from several locations. In the undeformed granites, the fine-grained quartz aggregates that formed from coesite upon decompression are characterized by a foam structure and random crystallographic orientation. In the deformed granites, the quartz microstructures and the crystallographic preferred orientation (CPO) indicate deformation by dislocation creep. Most of the deformation of the granites (if not all) must have happened at a late stage during exhumation, after transformation of coesite to quartz, at greenschist facies conditions in the middle crust. The deformed granites provide no evidence of deformation during subduction, at (U)HP metamorphic conditions, and in the earlier stages of exhumation. The diameter of internally undeformed slices of continental crust subducted to and exhumed from about 100 km can exceed that of the presently exposed Brossasco granite, i.e. it can be on the kilometre scale.     

Molybdenite Re–Os dating constrains gravitational collapse of the Sveconorwegian orogen, SW Scandinavia

Re–Os dating of molybdenite from small deposits is used to define crustal domains exhibiting ductile versus brittle behaviour during gravitational collapse of the Sveconorwegian orogen in SW Scandinavia. A 1019 ± 3 Ma planar quartz vein defines a minimum age for brittle behaviour in central Telemark. In Rogaland–Vest Agder, molybdenite associated with deformed quartz and pegmatite veins formed between 982 ± 3 and 947 ± 3 Ma in the amphibolite-facies domain (three deposits) and between 953 ± 3 and 931 ± 3 Ma west of the clinopyroxene-in isograd (two deposits) in the vicinity of the 0.93–0.92 Ga Rogaland anorthosite complex. The data constrain the last increment of ductile deformation to be younger than 0.95 and 0.93 Ga in these two metamorphic zones, respectively. Molybdenite is the product of an equilibrium between biotite, oxide and sulfide minerals and a fluid or hydrated melt phase, after the peak of 1.03–0.97 Ga regional metamorphism. Molybdenite precipitation is locally episodic. A model for gravitational collapse of the Sveconorwegian orogen controlled by lithospheric extension after 0.97 Ga is proposed. In the west of the orogen, the Rogaland–Vest Agder sector is interpreted as a large shallow gneiss dome, formed slowly in two stages in a warm and structurally weak crust. The first stage at 0.96–0.93 Ga was associated with intrusion of the post-collisional hornblende–biotite granite suite. The second stage at 0.93–0.92 Ga, restricted to the southwesternmost area, was associated with intrusion of the anorthosite–mangerite–charnockite suite. Most of the central part of the orogen was already situated in the brittle upper crust well before 0.97 Ga, and did not undergo significant exhumation during collapse. In the east of the orogen, situated against the colder cratonic foreland, exhumation of high-grade rocks of the Eastern Segment occurred between 0.97 and 0.95 Ga, and included preservation of high-pressure rocks but no plutonism.     

Microstructural tectonometamorphic processes and the development of gneissic layering: a mechanism for metamorphic segregation

The Mary granite, in the East Athabasca mylonite triangle, northern Saskatchewan, provides an example and a model for the development of non-migmatitic gneissic texture. Gneissic compositional layering developed through the simultaneous evolution of three microdomains corresponding to original plagioclase, orthopyroxene and matrix in the igneous rocks. Plagioclase phenocrysts were progressively deformed and recrystallized, first into core and mantle structures, and ultimately into plagioclase-rich layers or ribbons. Garnet preferentially developed in the outer portions of recrystallized mantles, and, with further deformation, produced garnet-rich sub-layers within the plagioclase-rich gneissic domains. Orthopyroxene was replaced by clinopyroxene and garnet (and hornblende if sufficient water was present), which were, in turn, drawn into layers with new garnet growth along the boundaries. The igneous matrix evolved through a number of transient fabric stages involving S-C fabrics, S-C-C' fabrics, and ultramylonitic domains. In addition, quartz veins were emplaced and subsequently deformed into quartz-rich gneissic layers. Moderate to highly strained samples display extreme mineralogical (compositional) segregation, yet most domains can be directly related to the original igneous precursors. The Mary granite was emplaced at approximately 900 °C and 1.0 GPa and was metamorphosed at approximately 750 °C and 1.0 GPa. The igneous rocks crystallized in the medium-pressure granulite field (Opx–Pl) but were metamorphosed on cooling into the high-pressure (Grt–Cpx–Pl) granulite field. The compositional segregation resulted from a dynamic, mutually reinforcing interaction between deformation, metamorphic and igneous processes in the deep crust. The production of gneissic texture by processes such as these may be the inevitable result of isobaric cooling of igneous rocks within a tectonically active deep crust.     

P-A分形模型定量度量糜棱岩变形过程中石英微结构的变化

定量度量动力变质作用下组成糜棱岩矿物颗粒的不规则变化是一项挑战性的研究.以基于GIS的PA(周长-面积)模型定量地描述糜棱岩中具自相似性石英形态的不规则变化.研究实例是苏格兰西北地区Moine逆冲断裂带前缘变质程度不同的5种类型的糜棱岩.石英是糜棱岩的主要矿物成分,其镜下的显微图片经扫描转换成数字图像,再通过GIS技术转换成矢量图,并计算出每颗石英的周长和面积.研究结果表明,随变质程度的增加,其PA分形维数分别从1.20、1.28、1.38、1.46,逐步增加到1.60.这表明,随变质程度的增加,变形程度与动态重结晶作用加强,石英颗粒从规则到不规则.     

Contrasting P–T–t paths for Neoproterozoic metamorphism in MacRobertson and Kemp Lands, east Antarctica

Mineral equilibria modelling and electron microprobe chemical dating of monazite in granulite facies metapelitic assemblages from the MacRobertson Land coastline, Rayner Complex, east Antarctica, are consistent with an 'anticlockwise' Neoproterozoic P–T–t path. Metamorphism occurred at c. 990–970 Ma, achieving peak conditions of 850 °C and 5.6–6.2 kbar at Cape Bruce, and 900 °C and 5.4–6.2 kbar at the Forbes Glacier ∼50 km to the east. These peak metamorphic conditions preceded the emplacement of regionally extensive syntectonic charnockite. High temperature conditions are likely to have been sustained for 80 Myr by lithospheric thinning and repeated pluton emplacement; advection was accompanied by crustal thickening to maximum pressures of 6–7 kbar, followed by near-isobaric cooling. This P–T–t path is distinct from that of rocks in adjacent Kemp Land, ∼50 km to the west, where a 'clockwise' P–T–t path from higher- P conditions at c. 940 Ma may reflect the response of a cratonic margin displaced from the main magma flux. In this scenario, crustal shortening was initially accommodated in younger, fertile crust (MacRobertson Land) involving metasediments and felsic plutons with the transfer of strain to adjacent older crust (Kemp Land) subsequent to charnockite emplacement.     

The recognition of former melt flux through high‐strain zones

High‐strain zones are potential pathways of melt migration through the crust. However, the identification of melt‐present high‐strain deformation is commonly limited to cases where the interpreted volume of melt “frozen” within the high‐strain zone is high (>10%). In this contribution, we examine high‐strain zones in the Pembroke Granulite, an otherwise low‐strain outcrop of volcanic arc lower crust exposed in Fiordland, New Zealand. These high‐strain zones display compositional layering, flaser‐shaped mineral grains, and closely spaced foliation planes indicative of high‐strain deformation. Asymmetric leucosome surrounding peritectic garnet grains suggest deformation was synchronous with minor amounts of in situ partial melting. High‐strain zones lack typical mylonite microstructures and instead display typical equilibrium microstructures, such as straight grain boundaries, 120° triple junctions, and subhedral grain shapes. We identify five key microstructures indicative of the former presence of melt within the high‐strain zones: (a) small dihedral angles of interstitial phases; (b) elongate interstitial grains; (c) small aggregates of quartz grains with xenomorphic plagioclase grains connected in three dimensions; (d) fine‐grained, K‐feldspar bearing, multiphase aggregates with or without augite rims; and (e) mm‐ to cm‐scale felsic dykelets. Preservation of key microstructures indicates that deformation ceased as conditions crossed the solidus, breaking the positive feedback loop between deformation and the presence of melt. We propose that microstructures indicative of the former presence of melt, such as the five identified above, may be used as a tool for recognising rocks formed during melt‐present high‐strain deformation where low (<5%) volumes of leucosome are “frozen” within the high‐strain zone.     

Fluid-assisted fracturing,cataclasis, and resulting plastic flow in mylonites from the Moresby Seamount detachment,Woodlark Basin

The Moresby Seamount detachment (MSD) in the Woodlark Basin (offshore Papua New Guinea) is a large active low-angle detachment excellently exposed at the seafloor, and cutting through mafic metamorphic rocks. Hydrothermal infiltration of quartz followed by that of calcite occurred during cataclastic deformation. Subsequent deformation of these a priori softer minerals leads to mylonite formation in the MSD. This study aims at a better understanding of the deformation mechanism switch from cataclastic to plastic flow. Deformation fabrics of the fault rocks were analyzed by light-optical microscopy. Rheologically critical phases were mapped to determine distributions and area proportions, and EBSD was used to measure crystallographic preferred orientation (CPO). Strong calcite CPOs indicate dominant dislocation creep. Quartz CPOs, however, are weak and more difficult to interpret, suggesting at least some strain accommodation by diffusion creep mechanisms. When quartz aggregates are intermixed with the polymineralic mylonite matrix diffusion creep grain boundary sliding may be dominant. The syntectonic conversion from mafic cataclasites to more siliceous and carbonaceous mylonites induced by hydrothermal processes is a critical weakening mechanism enabling the MSD to at least intermittently plastic flow at low shear stresses. This is probably a crucial process for the operation of low-angle detachments in hydrated and dominantly mafic crust.     

海南岛古元宙变质基底性质和地壳增生的Nd、Pb同位素制约

基于海南地壳各类型岩石的63个样品Nd和Pb同位素分析数据,研究了海南地块元古宙地壳变质基底的时代、特征和演化。研究结果表明,海南岛元古宙变质基底成熟度低,基底变质岩系的母岩物质来源于长期亏损的地幔源区,主要形成时代为古元古宙晚期－新元古宙;不同时代花岗岩具有较高的εNd(t)值和较低的Nd模式年龄,主要形成于幔源物质参与下的或含地幔成分较多的初生地壳再循环。地壳增生具幕式增生的特点,并在2．0Ga、1.7Ga、1.2Ga出现高峰;Pb同位素组成既不同于扬子地块又不同于华夏地块,介于两地块之间,和Nd同位素特征具有一致或耦合关系。结合海南岛地质特征,初步认为不能单纯地将海南岛基底理解为华南地块统一南延部分或是华夏古陆的部分,可能为不同的构造块体。     

40Ar/39Ar Constraints on the tectonic history and architecture of the ultrahigh-pressure Sulu orogen

New ⁴⁰Ar/³⁹Ar ages are presented from the giant Sulu ultrahigh-pressure (UHP) terrane and surrounding areas. Combined with U-Pb ages, Sm-Nd ages, Rb-Sr ages, inclusion relationships, and geological relationships, they help define the orogenic events before, during and after the Triassic collision between the Sino–Korean and Yangtze Cratons. In the Qinling microcontinent, tectonism occurred between 2.0 and 1.4 Ga. The UHP metamorphism occurred in the Yangtze Craton between 240 and 222 Ma; its thermal effect on the Qinling microcontinent was limited to partial resetting of K-feldspar ⁴⁰Ar/³⁹Ar ages. Subsequent unroofing at rates of 5–25 km Myr⁻¹ brought the UHP terrane to crustal levels where it underwent a relatively short amphibolite facies metamorphism. The end of that metamorphism is marked by ⁴⁰Ar/³⁹Ar ages in the 219–210 Ma range, implying cooling at crustal depths at rates of 50–200 °C Myr⁻¹. Ages in the 210–170 Ma range may reflect protracted cooling or partial resetting by Jurassic or Cretaceous magmatism. Jurassic 166–149 Ma plutonism was followed by cooling at rates of c. 15 °C Myr⁻¹, suggesting relatively deep crustal conditions, whereas Cretaceous 129–118 Ma plutonism was succeeded by cooling at rates of c. 50 C Myr⁻¹, suggesting relatively shallow crustal depths.     

U–Pb and Hf isotopic analysis of zircon in lower crustal xenoliths from the Navajo volcanic field: 1.4 Ga mafic magmatism and metamorphism beneath the Colorado Plateau

Zircon from lower crustal xenoliths erupted in the Navajo volcanic field was analyzed for U–Pb and Lu–Hf isotopic compositions to characterize the lower crust beneath the Colorado Plateau and to determine whether it was affected by ∼1.4 Ga granitic magmatism and metamorphism that profoundly affected the exposed middle crust of southwestern Laurentia. Igneous zircon in felsic xenoliths crystallized at 1.73 and 1.65 Ga, and igneous zircon in mafic xenoliths crystallized at 1.43 Ga. Most igneous zircon has unradiogenic initial Hf isotopic compositions (ɛ_Hf=+4.1–+7.8) and 1.7–1.6 Ga depleted mantle model ages, consistent with 1.7–1.6 Ga felsic protoliths being derived from “juvenile” Proterozoic crust and 1.4 Ga mafic protoliths having interacted with older crust. Metamorphic zircon grew in four pulses between 1.42 and 1.36 Ga, at least one of which was at granulite facies. Significant variability within and between xenoliths in metamorphic zircon initial Hf isotopic compositions (ɛ_Hf=−0.7 to +13.6) indicates growth from different aged sources with diverse time-integrated Lu/Hf ratios. These results show a strong link between 1.4 Ga mafic magmatism and granulite facies metamorphism in the lower crust and granitic magmatism and metamorphism in the exposed middle crust.     

Application of the titanium-in-quartz thermometer to pelitic migmatites from the Adirondack Highlands, New York

The 'TitaniQ' (Ti-in-quartz) solubility thermometer was applied to migmatitic metapelites from the southern and western Adirondack Highlands, New York, to examine the effect of granulite facies metamorphism on the distribution of Ti in quartz. Both cathodoluminescence imaging and quantitative traverses revealed that individual grains of Adirondack quartz are highly zoned with respect to Ti, and that core-to-rim decreases of Ti are common. Large ranges in calculated temperature were observed within each sample. One sample, not considered to be saturated with respect to TiO₂, gave maximum temperatures more than 100 °C lower than previously estimated peak temperatures. Rutile-saturated southern and western Adirondack samples yielded peak estimates of ≥803 ± 11 °C and ∼860–870 °C, respectively, which are similar to previous estimates from major phase thermometry. Minimum Ti-in-quartz matrix temperatures from rutile-saturated samples are 630 °C, which is interpreted as the closure temperature for Ti diffusion in quartz in these samples. This study demonstrates that Ti-in-quartz thermometry can yield details of rock evolution if the textural setting and reaction history of the quartz is clear, and can yield near-peak metamorphic temperatures in some cases, if care is taken to test for post-peak diffusional resetting.     

广东省河台金矿区糜棱岩锆石LA-ICP-MS U-Pb年龄及其地质意义

河台金矿区位于广东省高要市境内,普遍认为其是典型的韧性剪切带型金矿。本次对采自韧性剪切带中的糜棱岩和初糜棱岩样品进行了LA-ICP-MS锆石U-Pb定年,以约束剪切带的活动时代,并为金矿化事件提供依据。利用糜棱岩中的热液锆石约束韧性剪切带的变质变形时间,获得两组年龄:糜棱岩中热液锆石增生边加权平均年龄约为240Ma,代表左旋运动的年龄;初糜棱岩中热液锆石加权平均年龄约为204Ma,代表右旋运动的年龄。因此,矿区在印支期可能经历过两期剪切变形,并且这两期变形事件在整个华南都是普遍存在的。结合前人对矿化时间的研究,河台金矿的成矿时间(燕山期)要晚于韧性剪切带的形成时间(印支期)。样品中继承性锆石(糜棱岩中核部锆石)年龄显示云开群最晚沉积时间为早古生代早期,而并非前寒武地层。另外,本次研究还测试了锆石的Lu-Hf同位素,实验表明,可以根据热液锆石与继承性锆石的Lu-Hf同位素关系,判定热液锆石的形成条件。云开群的成岩物质主要来自1.8Ga地壳增生事件所形成的地壳物质发生重熔而产生的岩浆岩。     

Strain partitioning in the mid-crust of a transpressional shear zone system: Insights from the Homestake and Slide Lake shear zones,central Colorado

Kinematic analysis and field mapping of the Homestake shear zone (HSZ) and Slide Lake shear zone (SLSZ) in central Colorado may provide insight into the interaction between subvertical and low-angle shear zones in the middle crust. The northeast-striking, steeply dipping HSZ comprises a ∼10-km-wide set of anastomosing ductile shear zones and pseudotachylyte-bearing faults. Approximately 4 km south of the HSZ, north–northeast-striking, shallowly dipping mylonites of the SLSZ form three 1–10-m-thick splays. Oblique stretching lineations and shear sense in both shear zones record components of dip-slip (top-up-to-the-northwest and top-down-to-the-southeast) and dextral strike-slip movement during mylonite development. Quartz and feldspar deformation mechanisms and quartz [c] axis lattice preferred orientation (LPO) patterns suggest deformation temperatures ranging from ∼280–500 °C in the HSZ to ∼280–600 °C in the SLSZ. Quartz [c] axis LPOs suggest plane strain general shear across the shear system. Based on the relative timing of fabric development, compatible kinematics and similar deformation temperatures in the SLSZ and the HSZ, we propose that both shear zones formed during strain localization and partitioning within a transpressional shear zone system that involved subvertical shuffling in the mid-crust at 1.4 Ga.     

Cretaceous high-P granulites at Milford Sound, New Zealand: metamorphic history and emplacement in a convergent margin setting

Granulite facies orthogneiss of the Arthur River Complex (ARC) at Milford Sound, western Fiordland records a complex Early Cretaceous magmatic and orogenic history for the Pacific Gondwana margin that culminated in the emplacement and burial of a dioritic batholith, the Western Fiordland Orthogneiss (WFO). Enstatite-bearing mafic to intermediate protoliths of the ARC and WFO intruded the middle to upper crust. The early deformation history of the ARC is preserved in the Pembroke Granulite, where two-pyroxene S1 assemblages that reflect P <8 kbar and T  >750 °C were only patchily recrystallized during later deformation. S1 is cut by garnet-bearing, leucogabbroic to dioritic veins, which are cut by distinctive D2 fractures involving anorthositic veins and garnet–diopside–plagioclase-bearing reaction zones. These zones are widespread in the ARC and WFO and record conditions of P ≈14 kbar and T  >750 °C. Garnet–clinopyroxene-bearing corona reaction textures that mantle enstatite in both the ARC and WFO reflect Early Cretaceous burial by approximately 25 km of continental crust. Most of the ARC is formed from the Milford and Harrison Gneisses, which contain steeply dipping S4 assemblages that envelop the Pembroke Granulite and involve garnet, hornblende, diopside, clinozoisite, rutile and plagioclase, with or without kyanite. The P–T history of rocks in western Fiordland reflects pronounced Early Cretaceous convergence-related tectonism and burial, possibly related to the collision and accretion of island arc material onto the Pacific Gondwana margin.     

The assemblage diaspore+quartz in metamorphic rocks: a petrological, experimental and thermodynamic study

The upper pressure limit of pyrophyllite is given by the equilibria (i) pyrophyllite=diaspore+quartz and (ii) pyrophyllite=diaspore+coesite. High- P experimental investigations carried out to locate equilibrium (i) yield brackets between 497 °C/24.8  kbar and 535 °C/25.1  kbar, and between 500 °C/23  kbar and 540 °C/23  kbar. Equilibrium (ii) was bracketed at 550 °C between 26.0 and 28.3  kbar. In the experimental P–T  range, equilibria (i) and (ii) are metastable with respect to kyanite. A stable P–T  grid is calculated using thermodynamic data derived under consideration of the present experimental results. According to these data, the lower pressure limit of the assemblage diaspore+quartz according to equilibrium (i) range from about 12  kbar/300 °C to 20  kbar/430 °C (in the presence of pure water). The upper stability of diaspore+quartz is limited by the reaction diaspore+quartz=kyanite+H₂O at about 450 °C (nearly independent of pressure) and, to higher pressure, by the quartz=coesite transition. Equilibrium (ii) is metastable over the whole P–T  range.
Natural occurrences600.S of the diaspore–quartz assemblage in metamorphic rocks in Sulawesi, New Caledonia, Amorgos and the Vanoise are characterized by minerals indicative of high- P such as ferro-magnesiocarpholite, glaucophane, sodic pyroxene and lawsonite. The metamorphic P–T  conditions of these rocks are estimated to be in the range 300–400 °C, >8  kbar. These data are compatible with the derived P–T  stability field of the diaspore+quartz assemblage. We conclude that, in metamorphic rocks, diaspore+quartz is, as ferrocarpholite, an indicator for unusual low- T  /very high- P settings.     

Microstructures documenting Cenozoic extension processes in the northern continental margin of the South China Sea

ABSTRACT

In order to investigate the thinning process of the northern continental margin of the South China Sea, petrographic and microstructural analysis were carried out on 20 greenschistfacies mylonite samples, which were obtained from Site U1504 of IODP Expedition 367/368 in the Outer Margin High of the region. The mineral assemblage of the greenschist-facies mylonite is chlorite + epidotite + albite (Ab = 94.7–99.9) + quartz, which contains 10-30% gravel components. Microstructural analysis indicates that the greenschist-facies mylonite experienced two episodes of deformation：early ductile deformation followed by a later stage of brittle deformatio. Both episodes of deformation suggest an extensional environment. The extensive development of bulging recrystallization (BLG) of quartz, microscopic fractures and fine granulation of albite suggest that the temperature of ductile deformation is about 300-400°C, compatiable with a ductile shearing at shallow crust levels (~5-10 km). Petrographic features suggest that the greenschist-facies mylonite might originate from volcanic sedimentary rocks or sedimentary rocks affected by the intrusion of mafic magma. Combined with seismic interpretation, we propose that the greenschist-facies mylonite might be formed by crustal exhumation after thick Mesozoic sediments were denuded by a major extension.