SHRIMP U–Pb zircon dating of the host rocks of the Cannington Ag–Pb–Zn deposit,southeastern Mt Isa Block,Australia

SHRIMP U–Pb zircon ages are reported from a paragneiss, a pegmatite, a metasomatised metasediment and an amphibolite taken from the upper amphibolite facies host sequence of the Cannington Ag–Pb–Zn deposit at the southeastern margin of the Proterozoic Mt Isa Block. Also reported are ages from a middle amphibolite‐facies metasediment from the Soldiers Cap Group approximately 90 km north of Cannington. The predominantly metasedimentary host rocks of the Cannington deposit were eroded from a terrane containing latest Archaean to earliest Palaeoproterozoic (ca 2600–2300 Ma) and Palaeoproterozoic (ca 1750–1700 Ma) zircon. The ca 1750–1700 Ma group of zircons are consistent with sedimentary provenance from rocks of Cover Sequence 2 age that are now exposed to the north and west of the Cannington deposit. The metasedimentary samples also include a group of zircon grains at ca 1675 Ma, which we interpret as the maximum depositional age of the sedimentary protolith. This is comparable to the maximum depositional age of the metasediment from the Maronan area (ca 1665 Ma) and to previously published data from the Soldiers Cap Group. Metamorphic zircon rims and new zircon grains grew at 1600–1580 Ma during upper amphibolite‐facies metamorphism in metasedimentary and mafic magmatic rocks. Zircon inheritance patterns suggest that sheet‐like pegmatitic intrusions were most likely derived from partial melting of the surrounding metasediments during this period of metamorphism. Some zircon grains from the amphibolite have a morphology consistent with partially recrystallised igneous grains and have apparent ages close to the metamorphic age, although it is not clear whether these represent metamorphic resetting or crystallisation of the magmatic protolith. Pb‐loss during syn‐ to post‐metamorphic metasomatism resulted in partial resetting of zircons from the metasomatised metasediment.     

Eutectoid and cotectoid petrographic types of acid volcanic and subvolcanic rocks of the continental margin volcanic belts as exemplified by East Asia

Using continental-margin volcanic belts of East Asia as an example, the petrography and geochemistry of acid volcanic and subvolcanic rocks are considered. Cotectoid and eutectoid types are distinguished among them. The eutectoid type is characterized by a disequilibrium mineralogical composition, which includes phenocrysts of early “gabbro” paragenesis (pyroxenes, calcic plagioclase, occasional olivine and amphibole) and “granite” paragenesis (K-Na feldspar, quartz, biotite) and groundmass corresponding to dry quartz-feldspathic eutectics. A linear correlation is noted between the amount of phenocrysts of gabbro paragenesis and SiO₂ content. The entire petrochemical diversity of the eutectoid-type rocks is determined by the quantitative proportions between the gabbro paragenesis and bulk composition. The cotectoid rocks are dominated by sparsely-phyric and aphyric rocks lacking melanocratic inclusions and xenogenic minerals. In the cotectoid-type rocks, the compositions of the phenocrysts and groundmass are complementary with the bulk rocks composition, indicating their equilibrium cotectic crystallization.     

湖南香花岭锡铍多金属矿区的含Li、Be条纹岩和有关交代岩

湖南香花岭锡铍多金属矿床的含Li、Be条纹岩和有关交代岩,在世界上独一无二,产于燕山期(155~154 Ma)富氟花岗岩接触带及岩体的围岩捕掳体中,围岩为中上泥盆统灰岩和白云质灰岩。按矿物共生组合特征,条纹岩可大致分为铁锂云母条纹岩、氟硼镁石条纹岩、金绿宝石条纹岩、粒硅镁石-磁铁矿条纹岩和金云母-绿泥石条纹岩5类。与条纹岩有关的交代岩生成稍晚,可大致分为香花石交代岩、金云母交代岩、粒硅镁石交代岩、符山石-磁铁矿交代岩、金云母-绿泥石交代岩和韭闪石交代岩6类。文章对条纹岩和有关交代岩的生成地质背景、矿物共生组合和岩石地球化学特征进行了探讨。     

南岭地区新元古代花岗岩的岩石成因及构造意义

南岭地区新元古代花岗岩表现出了一定的共性:片麻状构造和过铝质特征.南岭西段岩体可归属于S型花岗岩,主要源自基底变沉积岩石的部分熔融,成岩过程中有少量幔源组分的参与;东段岩体为铝质A型花岗岩,源自还原性的长英质火成岩,有变沉积物的参与.这些花岗岩可能形成于“短期的地幔柱活动+长时限的俯冲”的构造背景中.     

Geochemical studies in the proterozoic metamorphic terrane of the guaxupé massif,minas gerais,brazil. A discussion on large ion lithophile element fractionation during high-grade metamorphism

The trace element composition of high-grade metamorphic rocks in the Guaxupé Massif is investigated. The large ion lithophile element composition is compared in both the granulite facies rocks and the associated amphibolite facies gneisses. The K, Rb, Th contents in the granulite facies rocks from other parts of Brazil show that the depletion of these elements is variable. The covariance of KRb ratios with K suggest the importance of mineralogical control, protolith composition, nature of fluid etc. The distribution of U and Th appears to indicate the protolith composition of the rock types.     

鞍山-本溪-抚顺地区新太古代地质演化探讨

鞍山-本溪-抚顺地区是我国太古宙岩石出露的主要地区及重点研究区.区内太古宙岩石以变质深成侵入体为主,变质表壳岩出露面积不足20%.区内变质深成岩侵位于始太古代-新太古代的不同时期,原岩由TTG岩系和钙碱性CA岩系组成;变质表壳岩以磁铁石英岩和斜长角闪岩为标志性岩性.两类原岩建造在太古宙时期,遭受了由麻粒岩相到角闪岩相至绿片岩相连续降温减压过程.根据区内新太古代岩石出露的特点划分3个微古陆核,分别为鞍本古陆核、清原古陆核及辽北古陆核.中太古代和新太古代为本区最重要的造陆时期,以鞍本微古陆核为最早的增生点,经拼接增生、裂解改造,形成现今的地质格局.     

青藏高原东南缘“三江”地区含角闪石变质岩石的地球化学特征

青藏高原东南缘"三江"地区变质杂岩带中的岩石类型十分复杂,斜长角闪岩和含角闪石斜长片麻岩是本区常见的变质岩石类型。斜长角闪岩可进一步划分为由石榴辉石岩退变而成的和基性原岩经角闪岩相-麻粒岩相变质形成的两种类型。斜长角闪岩的SiO2平均含量50.0%,具有高铝低钛的特点,轻稀土元素相对富集,铕无异常或略具负异常,Zr含量较低,Zr/Y比值较小,其原岩的主微量元素组成具有岛弧玄武岩的特点,反映其原岩形成于俯冲碰撞的构造环境。含角闪石斜长片麻岩的SiO2平均含量63.64%,MgO和TiO2含量低,轻、重稀土元素分馏,轻稀土元素明显富集而重稀土元素相对亏损,具有中等负Eu异常,大离子亲石元素K、Rb、Ba、Th强烈富集,高场强元素Ti、Zr、Hf、Nb和Ta含量较低,其原岩具有安山质岩石或安山质/玄武质岩石的特点,主要形成于板块聚敛边缘的构造环境。     

新疆天山成矿带铁磷矿地质及成矿专属性

本文对成矿带内的岩浆岩杂岩体进行了含矿性研究,划分出4个含矿杂岩带与5个矿床成因类型。成矿专属性研究表明:岩浆岩型铁磷矿受古地台边缘深大断裂控制;容矿岩石为主要偏碱性-铁质及铁镁质岩系列;容矿岩石主微量元素明显不和谐;岩石类型共生组合主要表现为黑云母透辉石岩、辉石岩、碱性辉长岩、斜长岩、碳酸岩的共伴生;矿物共生组合主要表现为透辉石+磷灰石+磁铁矿+黑云母,斜长石+磷灰石+磁铁矿+钠黝帘石组合,辉石+磷灰石+磁铁矿等特征;元素的共生组合主要表现为P、Fe、Ti、V、REE具显著相关性。通过预测研究,进行了找矿远景区的划分,划分出具有成矿条件的9个找矿远景区。     

利用定量矿物再造法恢复阜平群的原岩性质 介绍一种原岩恢复的新方法

恢复变质岩石的原岩性质,是在变质岩地区从事地质填图、矿产普查和勘探工作时所面临的重要而又困难的工作。目前人们利用野外地质产状、变余结构构造、岩石化学、地球化学、同位素地质学,副矿物等多种方法和手段来探讨这一问题,其中的一些方法已被广泛利用,有些方法在国内已有介绍。本文以阜平群为例介绍苏联学都鲁金科(B.     

Metamorphic history of glaucophane‐paragonite‐zoisite eclogites from the Shanderman area,northern Iran

The Shanderman eclogites and related metamorphosed oceanic rocks mark the site of closure of the Palaeotethys ocean in northern Iran. The protolith of the eclogites was an oceanic tholeiitic basalt with MORB composition. Eclogite occurs within a serpentinite matrix, accompanied by mafic rocks resembling a dismembered ophiolite. The eclogitic mafic rocks record different stages of metamorphism during subduction and exhumation. Minerals formed during the prograde stages are preserved as inclusions in peak metamorphic garnet and omphacite. The rocks experienced blueschist facies metamorphism on their prograde path and were metamorphosed in eclogite facies at the peak of metamorphism. The peak metamorphic mineral paragenesis of the rocks is omphacite, garnet (pyrope‐rich), glaucophane, paragonite, zoisite and rutile. Based on textural relations, post‐peak stages can be divided into amphibolite and greenschist facies. Pressure and temperature estimates for eclogite facies minerals (peak of metamorphism) indicate 15–20 kbar at ~600 °C. The pre‐peak blueschist facies assemblage yields <11 kbar and 400–460 °C. The average pressure and temperature of the post‐peak amphibolite stage was 5–6 kbar, ~470 °C. The Shanderman eclogites were formed by subduction of Palaeotethys oceanic crust to a depth of no more than 75 km. Subduction was followed by collision between the Central Iran and Turan blocks, and then exhumation of the high pressure rocks in northern Iran.     

Prograde transformations of amphibolites into eclogites and eclogite-like rocks in the low-pressure field of the eclogite facies (by the example of the Belomorian Mobile Belt)

Plagioclase-bearing garnet-omphacite (Grt-Omp) eclogites and garnet-augite eclogite-like (Grt-Aug) schists from the amphibolite and gneiss beds of the Belomorian Mobile Belt have been studied. They are spread over a large area. In most of the studied objects, these rocks have preserved primary concordant relations with the host amphibolite and gneiss strata; they are not disturbed by late tectonic processes and are not genetically related to tectonic-melange zones. Their protoliths were amphibolite lenses in gneisses or large mafic zones composed of amphibolites. The Grt-Omp eclogites formed in the low-pressure field of the eclogite facies (P = 12.5-13.0 kbar, T = 600-630 °C), and the eclogite-like Grt-Aug rocks, at the boundary between the amphibolite and eclogite facies (P = 9.6-11.1 kbar, T = 630-700 °C), under the intense impact of metamorphic fluid on the amphibolites. The compositional evolution of the rock-forming minerals during the formation of Grt-Omp eclogites and eclogite-like Grt-Aug rocks followed the same scheme. The petrographic diversity of apoamphibolite rocks (Grt-Omp eclogites and Grt-Aug schists) might be due to the difference both in the bulk composition of the metabasic protolith and in the ratios of CaO and Na₂O activities in the metamorphic fluid. The relatively low content of CaO leads to the formation of Grt-Omp paragenesis in eclogites. Higher CaO contents give rise to eclogite-like Grt-Aug rocks containing jadeite-poor clinopyroxene.     

Origin and metamorphic history of an Early Cretaceous polybaric granulite terrain,Fiordland, southwest New Zealand

Regionally extensive two-pyroxene granulites in Fiordland, southwest New Zealand, are products of metamorphism of a suite of anhydrous magmas which crystallized two pyroxenes. The granulite protolith (igneous charnockitic rock) synkinematically intruded metasediment and other orthogneiss in an Early Cretaceous subduction-related magmatic arc, and during cooling experienced deformation-induced recrystallization to form granoblastic gneiss. The granulites occur side by side with coeval rocks of amphibolite facies. Mineral zoning and textural relationships in both granulites and amphibolite facies rocks provide evidence of two distinct periods of crystallization: 1) an early high temperature, comparatively low pressure event accompanying magmatic intrusion (andalusite-sillimanite facies series recorded locally in the country rock), followed by 2) high pressure metamorphism under conditions of 650°–700° C at 12–13 kbar. Garnet granulite locally overprinted earlier formed two-pyroxene granulite during the latter event. The pressure increase (6 kbar) between the two events is attributed to crustal thickening by overthrusting, and is equivalent to unloading of a 20 km thick slab over rocks already buried at mid-crustal depths. Both events occurred over a < 20 m.y. interval, between the time of magmatic emplacement of the granulite protolith and uplift-controlled final cooling of the terrain. The Phanerozoic granulites in Fiordland share some petrologic similarities with Precambrian granulite terrains, suggesting that at least some aspects of the former may serve as a useful model for development of the latter.     

Provenance of the Sumian basal schists and age of the Lopian metavolcanic rocks at the Archean-Proterozoic boundary in the Kukasozero structure,North-Karelian zone of the Karelides,Baltic Shield

This paper presents the results of a study of the Paleoproterozoic basal garnet-kyanite-staurolite-two-mica paraschists from the Kukasozero structure of the Karelides of Northern Karelia, Baltic Shield, underlying Neoarchean acid metavolcanic rocks, and schists with quartz, phengite, kyanite, staurolite, garnet, and tschermakite located in the Paleoproterozoic rocks and considered to be metasomatic in origin. It was established that the sedimentary protolith of the Paleoproterozoic paraschists contains detritus of Neoarchean igneous rocks as follows from detrital 2737 ± 11 Ma zircons with oscillatory magmatic zoning. Metavolcanic 2757 ± 13 Ma rocks, close in age and composition, are known directly in the framework of the Kukasozero structure and are considered to be the most likely source of the sedimentary schist protolith. The coincidence of the Nd-model ages of paraschists (t _DM is 2.73–2.76 Ga) with the age of detrital zircons indicates no contribution of older rocks to the protolith composition. The age of magmatic crystallization of metavolcanic rocks directly underlying the Paleoproterozoic paraschists is 2681 ± 18 Ma and coincides with the age of porphyry granites in the western framework of the structure (2680.3 ± 3.6 Ma). No detrital zircons of similar age were found in basal paraschists, but the restricted amount of dated zircons does not allow us to draw a final conclusion about the absence of detritus of the underlying metavolcanic rocks in the paraschist protolith. It was confirmed that phengite-bearing schists are the products of acid metasomatism of the Paleoproterozoic amphibolites and amphibole schists (metavolcanic rocks). The metasomatic features were revealed in garnet-kyanite-staurolite-two-mica paraschists, so the strict identification of their sedimentary protolith is impossible. The paraschists do not represent metamorphosed weathering crust, because acid metasomatism gives a false impression of the greater maturity of the primary sedimentary rocks.     

Reactions to define the biotite isograd in the Ryoke metamorphic belt,Kii Peninsula,Japan

Two types of biotite isograd are defined in the low-grade metamorphism of the Wazuka area, a Ryoke metamorphic terrain in the Kii Peninsula, Japan. The first, BI₁, is defined by the reaction of chlorite+K-feldspar= biotite+muscovite+quartz+H₂O that took place in psammitic rocks, and the second, BI₂, by the continuous reaction between muscovite, chlorite, biotite and quartz in pelitic rocks. The Fe/Mg ratios of the host rocks do not significantly affect the reactions. From the paragenesis of pelitic and psammitic metamorphic rocks, the following mineral zones were established for this low-pressure regional metamorphic terrain: chlorite, transitional, chlorite-biotite, biotite, and sillimanite. The celadonite content of muscovite solid solution in pelitic rocks decreases systematically with the grade of metamorphism from 38% in the chlorite zone to 11% in the biotite zone. Low pressure does not prohibit muscovite from showing the progressive change of composition, if only rocks with appropriate paragenesis are chosen. A qualitative phase diagram of the AKF system relevant to biotite formation suggests that the higher the pressure of metamorphism, the higher the celadonite content of muscovite at BI₁, which is confirmed by comparing the muscovites from the Barrovian and Ryoke metamorphism.     

Evidence for a Variscan suture zone in the Vendée, France: a petrological study of blueschist facies rocks from Bois de Cené

Abstract In the Bois de Cené area, blueschist facies rocks, characterized by glaucophane and/ or chloritoid, provide evidence for a suture zone in the Variscan. This terrain is considered to be the eastern equivalent of the Ile de Groix high-pressure metamorphic terrain. Petrological study of the two characteristic types of rocks found in the area shows that the primary high-pressure paragenesis was modified during a retrogression which followed substantial decompression, probably at constant or decreasing temperature. The simplest interpretation is that this retrogression followed tectonic emplacement within a nappe pile.     

北苏鲁威海地区超基性岩的原岩形成时代和超高压变质时代

超基性岩是苏鲁超高压变质地体中一类特殊且十分重要的岩石类型,它们通常呈规模不一的块状、条带状或不规则透镜状 (体) 赋存于区域大面积出露的花岗质片麻岩中。锆石中矿物包体激光拉曼测试、阴极发光图像分析和不同性质锆石LA-ICP-MS U-Pb定年等综合研究结果表明,北苏鲁威海地区含橄榄石辉石岩 (样品W1和W2) 中锆石的成因十分复杂,可进一步划分3种不同类型锆石。其中第一类锆石呈自形-半自形晶,阴极发光图像显示清晰的岩浆结晶环带,矿物包体主要为Ol+Cpx+Ap, 记录的²⁰⁷Pb/²⁰⁶Pb年龄为1835~1845Ma,应代表含橄榄石辉石岩的原岩形成时代;第二类为变质重结晶锆石,呈半自形-他形晶,阴极发光图像显示模糊的岩浆结晶环带,矿物包体与第一类完全一致,记录的²⁰⁶Pb/²³⁸U年龄变化范围大,为250~784Ma之间,表明部分继承性岩浆结晶锆石明显受到后期岩浆-变质热事件的影响而发生不完全重结晶和Pb丢失,进而使其记录的年龄相对偏新;第三类锆石呈他形晶,为典型的变质锆石,阴极发光图像十分均匀,矿物包体相对少见,主要为Grt+Cpx,记录的²⁰⁶Pb/²³⁸U年龄为230~234Ma, 且与苏鲁地体榴辉岩及其围岩中含柯石英锆石微区记录的超高压变质年龄 (225~235Ma) 十分一致,应代表含橄榄石辉石岩的峰期超高压变质时代。超基性岩中超高压变质锆石的准确识别表明苏鲁地体在峰期超高压变质阶段的确存在流体,流体的存在对超高压变质锆石的形成起着至关重要的作用。该项研究不仅准确厘定北苏鲁威海地区超基性岩的原岩形成时代和超高压变质时代,而且对于深入探讨苏鲁-大别超高压地体流体行为、演化规律及其水-岩相互作用机理具有重要的科学意义。     

Caesium as a possible discriminant between felsic rocks of different origins from eastern Australia

The use of Cs to discriminate between igneous rocks derived from an igneous’ protolith (I‐type) and those derived from a metasedimentary protolith (5‐type) is evaluated for selected eastern Australian granitic and felsic volcanic rocks. For this investigation a ‘flame emission’ technique has been developed and tested for the rapid determination of Cs in rock samples. In the Berridale Batholith, where the I‐type, 5‐type subdivision was first recognized, the Cs content of the granites clearly distinguished the two granite types. In the New England Batholith the leucocratic 5‐types have higher Cs abundances, in most cases, than the similar I‐type granites, but the Cs values of the more mafic 5‐type granites overlap those of the I‐type. Problems in assigning origins for volcanics are complicated by alteration, but Cs may be useful in distinguishing reworked tuffs from volcanic rocks in volcanic sequences. Caesium may also find application in distinguishing volcanics and sediments in Precambrian terrains.     

霍各乞铜多金属矿区热水沉积岩类初探

以绿片岩相为主的霍各乞矿区含矿地层内存在一些特殊的热水沉积岩。通过矿物学、岩石学、矿物共生组合以及地球化学特征判断，主要岩石类型包括硅质岩、含电气石岩、重晶石岩、透辉石、透闪石岩以及铁镁碳酸盐岩类等。这些岩石的存在和研究对于本矿床及狼山地区成矿作用和矿床成因的认识有重要启示意义。     

Initial Mineral Composition and Geochemical Signature of High-Alumina Gneisses in the Chupa Formation of the Belomorian Group,the Baltic Shield

The normative mineral composition of protolith for high-alumina gneisses in the Chupa Formation of the Belomorian Group (Baltic Shield) is presented. Based on protolith composition, index of maturity or chemical index of alteration (CIA), localization of data points of gneisses on discriminant diagrams, and several index ratios, it is suggested that the protolith of the studied rocks was composed of low-mature sediments (graywackes). The material delivered to the sedimentary paleobasin was derived from ultramafic, mafic, and intermediate rocks. Correlation between some elements (Rb versus K₂O, Ti and Ga versus Al, and Sr versus smectitic clay and plagioclase), typical of Phanerozoic sedimentary rocks, is also revealed in the studied gneisses.     

Prehnite-pumpellyite facies metamorphism in Precambrian basement rocks of S Sweden

Very small amounts of prehnite and pumpellyite have been found, often as lenses in biotite crystals, in otherwise quite ordinary quartzo-feldspathic gneisses and granitoid rocks forming common components of the Precambrian basement of S Sweden. It is concluded that both minerals belong to the same metamorphic paragenesis, which also includes quartz, K-feldspar, albite, epidote, colourless mica and chlorite. The presence of this paragenesis, in however slight amounts, signifies that the rocks bear the imprint of a phase of metamorphism belonging to the prehnite-pumpellyite facies.