Porphyroblast rotation: eppur si muove*?

In a number of recent papers, the theory has been postulated that porphyroblasts as a rule do not rotate with respect to geographical coordinates, and can be used to determine the original orientation of older foliations. Complex inclusion patterns in spiral garnets have even been used to advocate a new model of orogenesis, involving several alternating phases of horizontal shortening and extension. Critical assessment of the assumptions and data used to support the theory of irrotational porphyroblasts reveals numerous flaws. Millipede structures, used as proof for flow partitioning, can also form by other flow geometries. Evidence quoted to support irrotational behaviour of porphyroblasts is unsound. Porphyroblasts do occur in sets with a preferred orientation of the internal foliation trace, but these cannot be shown to represent original orientations. Microstructures which resemble truncation planes in spiral garnets are used as evidence that these structures developed by several phases of deformation and as proof for periodic extension and horizontal shortening in orogenesis. They can, however, also be explained by intermittent growth of a rotating porphyroblast during a single phase of deformation. Finally, porphyroblast sets in which orientation is a function of aspect ratio indicate that porphyroblast rotation with respect to kinematic axes does occur in at least some situations.     

Lack of porphyroblast rotation in the Otago schists, New Zealand: implications for crenulation cleavage development, folding and deformation partitioning

Porphyroblasts of garnet and plagioclase in the Otago schists have not rotated relative to geographic coordinates during non-coaxial deformation that post-dates their growth. Inclusion trails in most of the porphyroblasts are oriented near-vertical and near-horizontal, and the strike of near-vertical inclusion trails is consistent over 3000 km². Microstructural relationships indicate that the porphyroblasts grew in zones of progressive shortening strain, and that the sense of shear affecting the geometry of porphyroblast inclusion trails on the long limbs of folds is the same as the bulk sense of displacement of fold closures. This is contrary to the sense of shear inferred when porphyroblasts are interpreted as having rotated during folding.
Several crenulation cleavage/fold models have previously been developed to accommodate the apparent sense of rotation of porphyroblasts that grew during folding. In the light of accumulating evidence that porphyroblasts do not generally rotate, the applicability of these models to deformed rocks is questionable.
Whether or not porphyroblasts rotate depends on how deformation is partitioned. Lack of rotation requires that progressive shearing strain (rotational deformation) be partitioned around rigid heterogeneities, such as porphyroblasts, which occupy zones of progressive shortening or no strain (non-rotational deformation). Therefore, processes operating at the porphyroblast/matrix boundary are important considerations. Five qualitative models are presented that accommodate stress and strain energy at the boundary without rotating the porphyroblast: (a) a thin layer of fluid at the porphyroblast boundary; (2) grain-boundary sliding; (3) a locked porphyroblast/matrix boundary; (4) dissolution at the porphyroblast/matrix boundary, and (5) an ellipsoidal porphyroblast/shadow unit.     

P-T path determinations in the Tormes Gneissic Dome, NW Iberian Massif, Spain

The Tormes Gneissic Dome (TGD, NW sector of the Iberian Massif, Spain) is a high-grade metamorphic complex affected by a major episode of extensional deformation (D2). The syn-D2 P–T  path of the Lower Unit of the TGD was deduced from the analysis of reaction textures related to superimposed fabrics developed during exhumation, analysis of mineral zoning and thermobarometric calculations. It comprises an initial phase of decompression, determined using the tweequ thermobarometric technique, from 6.4–8.1 kbar at 735–750 °C (upper structural levels) and 7.2 kbar at 770 °C (lower structural levels) to 3.3–3.9 kbar and 645–680 °C. This evolution is consistent with the observed sequence of melting reactions and the generation of garnet- and cordierite-bearing anatectic granitoids. The later part of the syn-D2 P–T  path consisted of almost isobaric cooling associated with the thermal re-equilibration of the unit in the new structural position. This segment of the P–T  path is recorded by assemblages with And +Bt+Ms and Ms+ Chl +Ab related to the later mylonitic S2 fabrics, which indicate retrogression to low-amphibolite and greenschist facies conditions.     

Ultrahigh-pressure metamorphism and exhumation of garnet peridotite in Pohorje, Eastern Alps

New evidence for ultrahigh‐pressure metamorphism (UHPM) in the Eastern Alps is reported from garnet‐bearing ultramafic rocks from the Pohorje Mountains in Slovenia. The garnet peridotites are closely associated with UHP kyanite eclogites. These rocks belong to the Lower Central Austroalpine basement unit of the Eastern Alps, exposed in the proximity of the Periadriatic fault. Ultramafic rocks have experienced a complex metamorphic history. On the basis of petrochemical data, garnet peridotites could have been derived from depleted mantle rocks that were subsequently metasomatized by melts and/or fluids either in the plagioclase‐peridotite or the spinel‐peridotite field. At least four stages of recrystallization have been identified in the garnet peridotites based on an analysis of reaction textures and mineral compositions. Stage I was most probably a spinel peridotite stage, as inferred from the presence of chromian spinel and aluminous pyroxenes. Stage II is a UHPM stage defined by the assemblage garnet + olivine + low‐Al orthopyroxene + clinopyroxene + Cr‐spinel. Garnet formed as exsolutions from clinopyroxene, coronas around Cr‐spinel, and porphyroblasts. Stage III is a decompression stage, manifested by the formation of kelyphitic rims of high‐Al orthopyroxene, aluminous spinel, diopside and pargasitic hornblende replacing garnet. Stage IV is represented by the formation of tremolitic amphibole, chlorite, serpentine and talc. Geothermobarometric calculations using (i) garnet‐olivine and garnet‐orthopyroxene Fe‐Mg exchange thermometers and (ii) the Al‐in‐orthopyroxene barometer indicate that the peak of metamorphism (stage II) occurred at conditions of around 900 °C and 4 GPa. These results suggest that garnet peridotites in the Pohorje Mountains experienced UHPM during the Cretaceous orogeny. We propose that UHPM resulted from deep subduction of continental crust, which incorporated mantle peridotites from the upper plate, in an intracontinental subduction zone. Sinking of the overlying mantle and lower crustal wedge into the asthenosphere (slab extraction) caused the main stage of unroofing of the UHP rocks during the Upper Cretaceous. Final exhumation was achieved by Miocene extensional core complex formation.     

西藏雅鲁藏布江缝合带普兰地幔橄榄岩铬尖晶石成因研究

普兰蛇绿岩位于雅鲁藏布江缝合带西段,其中地幔橄榄岩由方辉橄榄岩、含单斜辉石方辉橄榄岩以及少量二辉橄榄岩及纯橄岩组成。尖晶石是地幔橄榄岩中常见的副矿物,可以作为重要的岩石学成因指示剂。在野外地质调查基础上,通过岩相观察、电子探针、尖晶石成分面分析、电子背反射衍射分析,可将普兰地幔橄榄岩铬尖晶石分为三类:第一类铬尖晶石呈自形,粒径较小(<100μm),或包裹于斜方辉石中,或杂乱分布于橄榄石和辉石之间,具有高Cr^#(>0.6)、低Mg^#(0.43~0.57)的特征,为部分熔融+玻安质熔体交代成因;第二类铬尖晶石呈半自形-他形,粒径较大(>100μm),常含有橄榄石、辉石包裹体,具有中Cr^#(0.17~0.42)、高Mg^#(0.63~0.77)的特点,主要受部分熔融作用影响;第三类铬尖晶石呈他形蠕虫状与辉石交生在一起构成后成合晶结构,粒径变化较大,具有低Cr#(0.17~0.28)、高Mg^#(0.67~0.77)的特点。EBSD分析结果显示尖晶石、辉石的结晶学优选方位(CPO)较为相似,表明为同一矿物分解而来,单斜辉石与大陆岩石圈地幔捕掳体中石榴子石的稀土元素对比表明构成后成合晶结构的辉石和铬尖晶石为具有大陆岩石圈地幔属性的高压石榴子石退变分解而成。综合分析表明:普兰蛇绿岩的地幔橄榄岩体在从石榴子石相深度上升过程中发生了石榴子石退变、岩石部分熔融及熔体渗透作用,岩体经历了威尔逊旋回初期的大陆裂谷阶段,主体经历了中-低程度的部分熔融,类似大洋中脊环境,局部受到了富硅、富镁玻安质熔体的影响。     

电子探针-电感耦合等离子体质谱法研究不同种类石榴石的稀土元素配分和矿物学特征

石榴子石是变质岩和岩浆岩中一种常见的硅酸盐矿物,其类质同象非常普遍。已有资料表明,不同成分的石榴子石的颜色颇为不同,但石榴子石的成分和颜色之间相互关系尚未进行系统研究和总结。本文应用电子探针、电感耦合等离子体质谱、X射线粉晶衍射、拉曼光谱、红外光谱和紫外可见吸收光谱等手段对常见的红色（G1）、橙色（G2）、绿色（G3）和褐红色（G4）石榴石进行了系统测试,旨在揭示石榴子石成分、结构和颜色的内在关系和变异规律,以期为不同地质体中产出的石榴子石矿物学特征的总结及地质应用提供依据。研究结果表明,G1、G4样品含有较多Fe元素（Fe³⁺：0.24%、0.24%;Fe²⁺：1.01%、0.89%）;G2样品含有较高的Mn元素（2.76%）;G3样品含有很高的Cr、V元素（3453×10^-6、1458×10^-6）。类质同象对石榴石的晶体结构产生影响,晶胞参数有较大差别,分别是a=11.530nm（G1）、11.563nm（G2）、11.849nm（G3）和11.470nm（G4）。石榴石中的微量元素和稀土元素对于示踪物源及形成过程具有很强的指示意义。石榴石中的稀土元素总量分布不均匀,LREE/HREE比值小于1,表现为重稀土元素富集,Eu/Eu^*比值小于1,为Eu负异常。所有样品的Ce异常均不明显。石榴石样品的拉曼光谱呈现出峰强和峰位的明显差异也反映了类质同象的存在：G1、G4在570nm处出现Fe³⁺电子跃迁吸收峰;G2在460nm和520nm附近出现Mn²⁺电子跃迁吸收峰;G3在690nm处出现Cr³⁺电子跃迁吸收峰。紫外可见吸收光谱特征显示,红色和褐红色样品出现在570nm处的Fe³⁺电子跃迁吸收峰,与其成分中含有大量Fe有关;橙色样品于460nm和520nm附近的特征吸收峰归属于Mn²⁺,对应其主要成分中大量的Mn;绿色样品690nm处出现强的吸收峰,由Cr³⁺跃迁产生,是微量元素Cr的存在所致。研究结果表明,石榴石的颜色与其成分和结构具有良好的对应关系。     

Depletion and enrichment processes in the lithospheric mantle beneath the Kola Peninsula (Russia): Evidence from spinel lherzolite and wehrlite xenoliths

A suite of spinel lherzolite and wehrlite xenoliths from a Devonian kimberlite dyke near Kandalaksha, Kola Peninsula, Russia, has been studied to determine the nature of the lithospheric mantle beneath the northern Baltic Shield. Olivine modal estimates and Fo content in the spinel lherzolite xenoliths reveal that the lithosphere beneath the Archaean–Proterozoic crust has some similarities to Phanerozoic lithospheric mantle elsewhere. Modal metasomatism is indicated by the presence of Ti-rich and Ti-poor phlogopite, pargasite, apatite and picroilmenite in the xenoliths. Wehrlite xenoliths are considered to represent localised high-pressure cumulates from mafic–ultramafic melts trapped within the mantle as veins or lenses. Equilibration temperatures range from 775 to 969 °C for the spinel lherzolite xenoliths and from 817 to 904 °C for the wehrlites.

Laser ablation ICP-MS data for incompatible trace elements in primary clinopyroxenes and metasomatic amphiboles from the spinel lherzolites show moderate levels of LREE enrichment. Replacement clinopyroxenes in the wehrlites are less enriched in LREE but richer in TiO₂. Fractional melt modelling for Y and Yb concentrations in clinopyroxenes from the spinel lherzolites indicates 7–8% partial melting of a primitive source. Such a volume of partial melt could be related to the 2.4–2.5 Ga intrusion of basaltic magmas (now metamorphosed to garnet granulites) in the lower crust of the northern Baltic Shield. The lithosphere beneath the Kola Peninsula has undergone several episodes of metasomatism. Both the spinel lherzolites and wehrlites were subjected to an incomplete carbonatitic metasomatic event, probably related to an early carbonatitic phase associated with the 360–380 Ma Devonian alkaline magmatism. This resulted in crystallisation of secondary clinopyroxene rims at the expense of primary orthopyroxenes, with development of secondary forsteritic olivine and apatite. Two separate metasomatic events resulted in the crystallisation of the Ti–Fe-rich amphibole, phlogopite and ilmenite in the wehrlites and the low Ti–Fe amphibole and phlogopite in the spinel lherzolites. Alternatively, a single metasomatic event with a chemically evolving melt may have produced the significant compositional differences seen in the amphibole and phlogopite between the spinel lherzolites and wehrlites. The calculated REE pattern of a melt in equilibrium with clinopyroxenes from a cpx-rich pocket is identical to that of the kimberlite host, indicating a close petrological relationship.     

大别山榴辉岩中石榴石的结构水:红外光谱分析

对大别山(包括南大别和北大别)几个典型产地榴辉岩中的石榴石进行了傅里叶变换红外光谱(FTIR)分析,结果显示所有石榴石颗粒都含有一定量的结构水,以OH-的形式存在,结构水含量w(H2O)最高可达1859×10-6。水含量在同一榴辉岩样品不同石榴石颗粒间表现出较大的差异,反映了原岩中水含量的不均一分布。与以前报道的南大别碧溪岭榴辉岩中石榴石情况相似,水含量的不均一性表明:①榴辉岩原岩经历的水-岩交换作用的方式是“隧道式”而非“弥散式”;②超高压变质过程中流体的活动性十分有限;③整个超高压板块俯冲-折返的过程十分快速。南大别石榴石水含量总体上高于北大别,这种差异可能与北大别所经历的高温麻粒岩相变质作用有关。     

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.     

西昆仑造山带中巴公路段石榴角闪岩形成的变质地质过程

西昆仑中巴公路剖面上多处产出石榴黑云角闪片岩、帘石石榴角闪岩和含榴斜长角闪片岩，即石榴角闪岩。不论在宏观尺度还是在微观尺度上中级变质的石榴角闪岩均可与较低级变质的黑云母—绿帘石—绿泥片岩呈条带状相间排列。通过野外岩石组合—构造变形的观察，结合室内岩石结构、变质组合和反应关系的分析，认为该区石榴角闪岩的形成与较低级变质基础上的局部变质叠加有关，这种叠加在构造活动的中心部位最为显著，石榴角闪岩的产出表明变质已达高角闪岩相。石榴子石—角闪石—斜长石的形成与一定程度上的流体活动密切相关，当流体挥发分组分活动强烈时，石榴子石不出现，而代之以单斜辉石—方柱石—斜长石组合。在组分活性方面，镁铁质组分比长英质组分的活动更为明显。石榴角闪岩的原岩既可以是基性岩，如玄武岩或辉长岩，也可以是沉积岩如杂砂岩。