The belt of metagabbros of La Pampa: Lower Paleozoic back-arc magmatism in south-central Argentina

Combined geological, geochronological, geochemical and geophysical studies have led to identification of a large (∼300 km long, ∼5 km wide) N–S trending belt of metagabbros in the province of La Pampa, south-central Argentina. This belt, though only poorly exposed in the localities of Valle Daza and Sierra de Lonco Vaca, stands out in the geophysical data (aeromagnetics and gravity). Modeling of the aeromagnetic data permits estimation of the geometry of the belt of metagabbros and surrounding rocks.The main rock type exposed is metagabbros with relict magmatic nucleii where layering is preserved. A counterclockwise P–T evolution affected these rocks, i.e., during the Middle Ordovician the protolith reached an initial granulite facies of metamorphism (M1), evolving to amphibolite facies (M2). During the Upper Devonian, a retrograde, greenschist facies metamorphism (M3) partially affected the metagabbros.The whole-rock Sm–Nd data suggest a juvenile source from a depleted mantle, with model ages ranging from 552 to 574 Ma, and positive Epsilon values of 6.51–6.82. A crystallization age of 480 Ma is based on geological considerations, i.e. geochronological data of the host rocks as well as comparisons with the Las Aguilas mafic–ultramafic belt of Sierra de San Luis (central Argentina).The geochemical studies indicate an enriched MORB and back-arc signature.The La Pampa metagabbros are interpreted to be originated as a result of the extension that took place in a back-arc setting coevally with the Famatinian magmatic arc (very poorly exposed in the western part of the study area). The extensional event was ´aborted´ by the collision of the Cuyania terrane with Pampia-Gondwana in the Middle Ordovician, causing deformation and metamorphism throughout the arc–back-arc region.The similarities between the La Pampa metagabbros and the mafic–ultramafic Las Aguilas belt of the Sierra de San Luis are very conspicuous, for example, the age (Lower Paleozoic), geochemical signature and timing of metamorphism (dated at ca. 465 Ma in the study area), which allow definition of a single, mafic back-arc belt in central Argentina, from San Luis to La Pampa.     

Petrogenesis of Eclogites in the Light of Punctuated Metamorphic Evolution in Dabie Terrane，China

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Cambrian to Devonian Geologic Evolution of the Sierra de Comechingones, Eastern Sierras Pampeanas, Argentina: Evidence for the Development and Exhumation of Continental Crust on the Proto-Pacific Margin of Gondwana

Crystalline rocks from the Sierra de Comechingones, eastern Sierras Pampeanas, evolved through three distinct orogenic cycles during the Eopalaeozoic: (1) the first tectono-thermal event named Pampean orogeny (550 to 505 Ma), which peaked in the Early Cambrian, was responsible for extensive metamorphism, partial melting, juvenile magmatism, rapid decompression, and persistent tectonic activity. Large part of the crustal section that was residing at middle levels (c. 27 km) was heated above 800 °C during the thermal peak stage of the Pampean orogeny; decompression of the Pampean orogen's core took place at this high temperature. The exhumation mechanism that assisted rapid uplifting combined the effects of ongoing tectonic forces with a buoyant instability created by a large amount of anatectic magmas in the middle to lower crust. (2) Beginning at the Early Ordovician, the Famatinian orogeny produced an overall shortening, causing pervasive textural reworking of the Cambrian metamorphic sequences under a high-strain regime. By being adjacent to the Famatinian magmatic arc, the western border of the Cambrian crystalline package absorbed imposed deformation along a crustal scale ductile shear zone. Within this zone, the high-grade metamorphic rocks were reworked and re-hydrated to lower temperature assemblages (<600°C and 3–6 kbar). Early Ordovician subduction-related igneous activity, even though manifested as small plutons, intruded Cambrian crystalline sequences, and experienced textural reworking during Late Famatinian tectonic exhumation. Late Famatinian convergence resulted in west-vergent ductile shear zones that placed Cambrian onto Ordovician crystalline sequences. (3) During post-Famatinian times (360–400 Ma) enduring crustal perturbation produced intra-crustal-derived granitic magmatism. West- to northwest-directed thrusting was concentrated in belts nucleated along crustal-scale tectonic boundaries formed between older tectono-stratigraphic units. As a result, Devonian anatectic granites were formed and tectonically extruded among Pampean and Famatinian crystalline sequences. The post-Famatinian event is also characterised by the intrusion of batholith-scale monzogranites into Pampean and Famatinian crystalline sequences residing in the upper crust.

Crystalline rocks currently exposed in the Sierra de Comechingones show that they crystallised and were exhumed in a setting where tectono-thermal activity lasted, even though it might have waned, until the Middle Palaeozoic. From the latest Neoproterozoic (c. 550 Ma) until the Late Devonian (c. 360 Ma) tectonic activity was intermittently acting, indicating continuous convergence along the proto-Pacific margin of Gondwana.     

Ultrahigh Pressure Metamorphism and Tectonic Evolution of Southwestern Tianshan Orogenic Belt,China: A Comprehensive Review

Recently, a huge ultrahigh‐pressure (UHP) metamorphic belt of oceanic‐type has been recognized in southwestern (SW) Tianshan, China. Petrological studies show that the UHP metamorphic rocks of SW Tianshan orogenic belt include mafic eclogites and blueschists, felsic garnet phengite schists, marbles and serpentinites. The well‐preserved coesite inclusions were commonly found in eclogites, garnet phengite schists and marbles. Ti‐clinohumite and Ti‐chondrodite have been identified in UHP metamorphic serpentinites. Based on the PT pseudosection calculation and combined U‐Pb zircon dating, the P‐T‐t path has been outlined as four stages: cold subduction to UHP conditions before ～320 Ma whose peak ultrahigh pressure is about 30 kbar at 500oC, heating decompression from the Pmax to the Tmax stage before 305 Ma whose peak temperature is about 600oC at 22kbar, then the early cold exhumation from amphibolite eclogite facies to epidote‐amphibolite facies metamorphism characterized by ITD PT path before 220 Ma and the last tectonic exhumation from epidote amphibolite facies to greenschist facies metamorphism. Combining with the syn‐subduction arc‐like 333‐326 Ma granitic rocks and 280‐260 Ma S‐type granites in the coeval low‐pressure and high‐temperature (LP‐HT) metamorphic belt, the tectonic evolution of Tianshan UHP metamorphic belt during late Cambrian to early Triassic has been proposed.     

Geochronology of metamorphism,deformation and fluid circulation: A comparison between Rb-Sr and Ar-Ar phyllosilicate and U-Pb apatite systematics in the Karagwe-Ankole Belt (Central Africa)

This work presents an integrated geochronology study combining step-heating Ar-Ar and in-situ Rb-Sr phyllosilicate geochronology with EPMA elemental maps and U-Pb apatite geochronology in order to determine the timing and duration of regional deformation and fluid circulation processes in the Karagwe-Ankole Belt in Central Africa. The Kibuye-Gitarama-Gatumba study area (West Rwanda) forms part of the Western Domain of this Mesoproterozoic belt, where the metamorphic and deformation history, geothermometric evolution and mineralization processes (both genesis and geochronology) have been well-documented in previous studies.Our geochronological results and interpretations show the importance of thermal diffusion throughout the elevated temperature regime present in the Neoproterozoic. During the Ediacaran, metamorphism and fluid alteration in the Gatumba area was associated with deformation, as independently demonstrated by U-Pb apatite and Ar-Ar phyllosilicate geochronology. From the end of the Ediacaran until the Late Cambrian, saline fluid activity is recorded and fluid-induced (re)crystallization processes dominate the Gatumba biotite isotope record. The direct comparison between and Ar-Ar geochronology suggests a parentless ⁴⁰Ar-component is preserved during this fluid-induced (re)crystallization due to its saline, (earth) alkali-rich composition. Additionally, this multi-method approach allows us to place the history of the Western Domain of the Karagwe-Ankole belt in relation to that of the surrounding areas, i.e. the Eastern Domain and the Congo and Tanzania Cratons. Deformation and epidote-amphibolite facies metamorphism (at ~590–570 Ma), and fluid fluxes (at ~590–495 Ma) occurred in the Kibuye-Gitarama-Gatumba area after the West Gondwana Orogeny but from the onset of the East African Orogeny onwards till after the culmination of the Gondwana amalgamation, demonstrating the progressively eastward-moving orogenic front in this supercontinent assembly.     

40Ar/39Ar evidence that formation of blueschists in continental crust was synchronous with foreland fold and thrust belt deformation, western Brooks Range, Alaska

Abstract ⁴⁰Ar/³⁹Ar ages from white mica in rocks of the internal zone of the Brooks Range contractional orogen indicate that the Nanielik antiformal duplex developed at about 120 Ma and was remobilized on its southern boundary at c . 108 Ma. Blueschist facies metamorphism accompanied development of the antiform. The timing of the blueschist facies event and creation of the antiform overlap the period of shallow-seated deformation in the foreland fold and thrust belt and sedimentation in the foreland basin of the Brooks Range. Blueschist facies P-T conditions may therefore characterize the thicker parts of orogenic wedges in some orogenic systems; ancient blueschists need not necessarily be interpreted as indicators of active subduction or continent-continent collision.
Microprobe analysis using quantitative wavelength-dispersive and electron backscattered electron imaging methods was used to characterize the composition of white micas in the dated samples. None of the samples was compositionally homogeneous; many contained 2-3 populations of white mica, including both potassic and sodic varieties. Samples which had undergone (in sequence) amphibolite, albite-epidote amphibolite and blueschist facies metamorphic events retained muscovites relict of the amphibolite facies event. Samples that had undergone only the blueschist facies event also contained multiple populations of mica, some probably from detrital sources.     

Crustal Provenance and Cooling of the Basement Complexes of the Sierra de San Luis: An Insight Into the Tectonic History of the Pro to-Andean Margin of Gondwana

The Sierra de San Luis constitutes the southernmost tip of the Eastern Sierras Pampeanas. Its Palaeozoic metamorphic basement units define a key location for the understanding of the accretional history along the proto-Andean margin of Gondwana. Although, it is largely accepted that the polyphase accretional history of the Sierras Pampeanas is preluded by the docking of the Pampean Terrane followed by the Famatinian Orogenic Cycle that involves subduction along the margin of Gondwana and the accretion of the Precordillera (Cuyania) Terrane and finally ceased with the collision of the Chilenia terrane, a vast amount of controversial information concerning the timing and mode of collisions as well as the origin of the different involved crustal fragments within the Eastern Sierras Pampeanas is published. In this paper, those different hypothesis are presented and evaluated under the light of new isotopic data of the Sierra de San Luis. Nd-systematics of the metasedimentary sequences of the Sierra de San Luis indicate that the studied sequences were developed on the Pampean Terrane. An Amazonian origin of the Pampean Terrane that was probably detached from the Arequipa Antofalla Craton is proposed. Furthermore, the correlation of two low-grade phyllitic belts (San Luis Formation) with the widespread Puncoviscana Formation is not supported by Sm-Nd data. It is suggested that the sedimentary precursors of the Pringles Metamorphic Complex and the topping phyllites were sourced on the Pampean Orogen and accommodated in a newly formed back arc basin during the early Famatinian.

The cooling history of the basement complex is recorded by an extensive amount of K-Ar muscovite and biotite ages. A high variability in muscovite ages is only partly related to different intrusion times of two pegmatoid generations. Post Famatinian to Achalian crustal scale mylonite formation (-359 Ma) and a rotational exhumation of the central basement unit are causal for the observed K-Ar muscovite age pattern in the range from 395 Ma to 447 Ma. Therefore, the decrease in metamorphic degree from west to east is the result of the erosion level of a crustal profile from the mid lower crust to the upper crust. An even higher variability in K-Ar biotite cooling ages covering the range from 315 Ma to 418 Ma is related to the slow cooling after the Famatinian Orogenic Cycle or reheating during the Achalian Orogenic Cycle and consequent variable reset of the isotopic system. However, ages recorded by biotite booklets substantiate the hypothesis of a differential exhumation of the basement of the Sierra de San Luis.     

桐柏秦岭岩群的两类变质作用

本文重点对河南桐柏地区的秦岭岩群进行了观察与研究,根据野外地质、岩相关系及同位素测年资料,提出该区秦岭岩群具有明显不同的两类变质作用,一是较早期的高温麻粒岩相变质作用,以包体或长透镜群、甚至巨型条块状局限于中部郭庄组的花岗质片麻岩之中。根据伟晶岩、片麻岩及麻粒岩锆石年龄的综合限定,该变质作用的时间可能为~498Ma,多数人主张的445~430Ma的麻粒岩相变质年龄实际上是早期锆石被后期岩浆或变质事件引起的同位素体系重启年龄。另一种是相对晚期的角闪岩相变质作用,变质程度以角闪岩相为主,局部达高角闪岩相,没有任何早期高温或高压变质的残留迹象,形成秦岭岩群中主导类型的变质作用。同样,采用伟晶岩及有关片麻岩和麻粒岩中锆石测年限定,角闪岩相变质时间可能为~472Ma。高温麻粒岩的产出具有其特殊机制,大量的花岗质岩浆侵位过程中把地壳深部的高温麻粒岩裹挟上升至浅部层次,随后一起遭受区域上的角闪岩相变质作用。     

Age Constraints on the Tectonic Evolution and Provenance of the Pie de Palo Complex, Cuyania Composite Terrane, and the Famatinian Orogeny in the Sierra de Pie de Palo, San Juan, Argentina

New U-Pb age determinations confirm earlier interpretations that the strongly deformed and metamorphosed mafic and intermediate igneous rocks of the Pie de Palo Complex represent a Mesoproterozoic fragment of suprasubduction zone oceanic crust.

A gabbroic pegmatite, interpreted to have formed during arc rifting or subsequent back-arc spreading, yielded a U-Pb age of 1204 ^+5.3/_–4.7 Ma. Highly tectonized ultramafic-mafic cumulates, occurring at the structural base of the Pie de Palo Complex and previously interpreted to represent remnants of a primitive arc phase, prior to rifting and back-arc spreading, could not be dated, but should be older than 1204 Ma if these inferences are correct. Tabular, sill-like bodies of leucogabbro/diorite and calc-alkaline tonalite/granodiorite sills yielded ages of 1174±43 and 1169 ⁺⁸/_–7 Ma respectively. They may represent a younger, more evolved arc phase established after arc rifting or a younger, tectonically unrelated Mesoproterozoic arc. SHRIMP-analysis of metamorphic zircon rims with low Th/U ratios in VVL 110 gave a ²⁰⁶Pb/²³⁸U age of 455±10 Ma, similar to lower intercept dates determined by discordia lines. Combined, these data indicate that the bulk of the amphibolite facies metamorphism present in the Pie de Palo Complex was generated during the Famatinian Orogeny.

Analysis of six single detrital zircon grains in a metasedimentary, quartzofeldspathic garnet-mica schist, tectonically interleaved with the igneous rocks of the Pie de Palo Complex, and tentatively correlated with the Difunta Correa metasedimentary sequence of other workers, yielded three age populations: 1150–1160 Ma; 1050–1080 Ma and 665 Ma, indicating that these sedimentary rocks were deposited during the late Neoproterozoic or Early Paleozoic. In addition, they confirm structural evidence that intercalation of rocks of the Pie de Palo Complex with isolated slivers of these sedimentary rocks is due to tectonic imbrications. These ages are also consistent with a Laurentian provenance, and earlier interpretations that these rocks once represented a sedimentary cover to the Pie de Palo Complex. The zircon population of 1050–1080 Ma could be derived from Grenville-age felsic plutons identified elsewhere in the Pie de Palo Complex by other workers. However, no evidence has been found in our samples for a Grenville-age orogenic event, invoked previously to explain accretion of the oceanic Pie de Palo Complex to Laurentia prior to the late Neoproterozoic/Early Cambrian rifting and drift of Cuyania.     

变质作用分类的历史回顾和新的试行分类雏议

根据变质作用分类研究的历史回顾和最新进展,本文介绍了一个试行的、较系统的变质作用分类方案,可分为局部性的变质作用和区域性的变质作用两个大类9个类型.局部性的变质作用包括以下5个类型:(1)接触变质作用,包括热变质作用、接触交代变质作用和高热变质作用3个亚类;(2)低-中-高温动力变质作用,包括低-中-高温韧性剪切带变质作用和温压递进逆冲断层变质作用2个亚类;(3)冲击变质作用;(4)水热变质作用;(5)燃烧变质作用.区域性的变质作用可分为以下4个类型:(6)早前寒武纪克拉通基底变质作用-低中压-中高温区域变质作用,可分为5个亚类,分别是低-中压角闪岩相变质作用、低-中压麻粒岩相变质作用、低-中压角闪岩相-麻粒岩相变质作用、低-中压绿片岩相-角闪岩相递增的变质作用和区域超高温麻粒岩相变质作用;(7)造山带变质作用,可分为3个亚类,分别是低温-低压区域动力变质作用、碰撞造山变质作用和深俯冲高压-超高压区域变质作用;(8)埋深变质作用;(9)洋底变质作用.     

胶北地块斜长角闪岩的岩石学与年代学研究

胶北地块位于华北克拉通东部陆块,胶-辽-吉活动带的南端.胶北地区荆山杂岩中存在一组与高压基性麻粒岩密切共生的斜长角闪岩,是构成前寒武纪变质基底的重要组成部分.岩石学矿物学研究表明,斜长角闪岩记录了3个阶段的变质作用:峰期变质矿物组合（M1）为角闪石1+斜长石+榍石,根据NCKFMASHTO体系的成分视剖面图和角闪石压力计估算出温度条件T=660~715 ℃,压力条件P=0.65~0.71 GPa;其后经历了退变质作用（M2）,矿物组合为角闪石2+绿帘石+斜长石+绿泥石+钠长石,估算温压条件为537~630 ℃/0.41~0.58 GPa;晚期发生前绿片岩相退变质作用（M3）,其矿物组合为钠长石+葡萄石+绿泥石+方解石,其温压条件 < 400 ℃/0.35 GPa.斜长角闪石的CL图像显示其具有较弱的阴极发光效应和弱震荡环带,Th/U比值相对较小（0.06~0.43）,锆石形态和内部结构指示锆石形成于深熔作用过程,21个锆石的LA-ICP-MS定年研究的结果表明,斜长角闪岩记录的最老206Pb/238U年龄为2 075±25 Ma,上交点年龄为1 845±23 Ma（MSWD=0.35）,该组年龄记录了斜长角闪岩峰期变质作用时代的上限.斜长角闪岩在原岩形成以后,可能曾经历麻粒岩相变质作用,并记录了在胶-辽-吉带~1.85 Ga碰撞闭合过程中的深熔事件,此后经历了角闪岩相变质作用,及其二次退变质作用,终结于前绿片岩相的变质温压条件.      

西昆仑布伦阔勒岩群中泥质高压麻粒岩的岩石学、年代学及其地质意义

西昆仑的深变质岩类主要发育于布伦阔勒岩群之中,其中的高压麻粒岩是西昆仑造山带中目前已知的变质程度最高的岩石.本文以其中的泥质高压麻粒岩为研究对象,结合岩相学、相平衡模拟以及锆石年代学分析等方法进行研究.结果 显示其峰期变质矿物组合蓝晶石+石榴石+钾长石,是典型的泥质高压麻粒岩岩石组合.根据相平衡模拟估算,高压麻粒岩相峰...     

胶北蓬莱群的变形与变质及其区域大地构造意义

本文通过构造与变质研究及伊利石结晶度和同位素年龄测定。分析胶北蓬莱群的变形,变质及其区域大地构造意义。作为胶北唯一盖层的蓬莱群,在晚石炭世(299±4Ma)北北东—南南西区域挤压下首次遭受了强烈的蓬莱事件变形及低绿片岩相变质。其后的后蓬莱事件在胶北地区仅造成了局部的北北东—南南西向变形构造。作为华北板块南缘的胶北带,其上的蓬莱事件是华北与扬子板块沿胶南带碰撞的结果。这表明郯庐断裂带以东这两扳块碰撞发生在晚石炭世。在秦岭—大别山碰撞带以北的华北扳块南缘,由于碰撞也出现了类似胶北带的挤压变形。     

Remnants of a Neoproterozoic Collisional Orogenic Belt in the Core of the Phanerozoic Qinling Orogenic Belt (China)

NW-SE trending paleotectonics are recognized from the WNW-ESE trending Qinling complex in the core of the Qinling belt. They consist of trending stratum, boundary shear zones, deformed linear plutons and ophiolite. The field relationships and ages of the deformation, magmatism and metamorphism suggest that the paleotectonics formed in Neoproterozoic. The ages of deformed and undeformed plutons approximately constrain the peak deformation during 958-889 Ma. Both the two trending tectonics constitute an orogen-scale tectonic replacement by S2 (Phanerozoic WNW-ESE tectonics) of S1 (NNW-SSE trending Neoproterozoic), similar to structural replacement in outcrop. The strong contractional deformation, (high-pressure) metamorphism, and granitic magmatism that evolved from syn-collisional (S-type), to post-collisional (I-type) and to post-collisional (A-type), as well as regional extension at 885-700 Ma in Qinling, display a cycle of a collisional orogeny. Thus, the NW-SE trending paleotectonics could be remnants of a collisional orogenic belt. The belt originally could trend NNW-SSE, if reworking and modification of Paleozoic and/or Mesozoic NNE-SSW contractional deformation are eliminated. Accordingly, assemblage and breakup of some continental blocks occurred in Neoproterozoic in Qinling, which seems to correspond to assemblage and breakup of Rodinia supercontinent in the world. This study provides new insight into the evolution of the Qinling orogenic belt and the reconstruction of Proterozoic supercontinent of China, and also presents a case study of recognition of an old orogen from a young orogen.     

Grenvillian-aged orogenesis in the Palaeoproterozoic Gascoyne Complex, Western Australia: 1030–950 Ma reworking of the Proterozoic Capricorn Orogen

In situ SHRIMP U–Pb geochronology of monazite and xenotime in pelitic schists from the central Gascoyne Complex, Western Australia, shows that greenschist to amphibolite facies metamorphism occurred between c. 1030 and c. 990 Ma. Monazite from an undeformed rare‐element pegmatite from the same belt gives a ²⁰⁷Pb/²⁰⁶Pb age of c. 950 Ma, suggesting that peak metamorphism and deformation was followed by pegmatite intrusion and coeval granite magmatism. Metamorphism in the central Gascoyne Complex was previously interpreted as Barrovian, largely based on the identification of kyanite in peak metamorphic assemblages, and has been attributed to intense crustal shortening and substantial tectonic thickening during Palaeoproterozoic continent–continent collision. However, the stable Al₂SiO₅ polymorph has been identified in this study as andalusite rather than kyanite, and the prograde assemblages of staurolite–garnet–andalusite–biotite–muscovite–quartz indicate temperatures of 500–550 °C and pressures of 3–4 kbar. These data show that the Palaeoproterozoic Gascoyne Complex underwent an episode of Grenvillian‐aged intracontinental reworking concentrated in a NW–SE striking corridor, during the Edmundian Orogeny. Until now, the Edmundian Orogeny was thought to have involved only reactivation of structures in the Gascoyne Complex, along with deformation and very low‐ to low‐grade metamorphism of Mesoproterozoic cover rocks some time between 1070 and 755 Ma. However, we suggest that it involved regional amphibolite facies metamorphism and deformation, granite magmatism and pegmatite intrusion between c. 1030 and c. 950 Ma. Therefore, the Capricorn Orogen experienced a major phase of tectonic reworking c. 600 Myr later than previously recognized. Our results emphasize the importance of in situ geochronology integrated with petrological studies in order to link the metamorphic history of a terrane with causally related tectonic events.     

A Comparison about Metamorphism among the Oldest-Rock Units from Orogenic Belts of Dabie, Eastern Qinling and Eastern Kunlun of Central Mountain Ranges, China

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吉林省和龙地区鸡南BIF型铁矿床含矿建造地球化学特征及形成时代

鸡南铁矿床位于吉林省和龙地区,地处华北克拉通北缘与兴蒙造山带接壤的龙岗地块北部,是东北地区发现较早的BIF型铁矿床之一。该矿床铁矿体主要呈层状、似层状、扁豆状赋存于鞍山群鸡南组上段中部层位,含矿岩石以黑云斜长片麻岩、角闪黑云斜长片麻岩、黑云角闪斜长片麻岩及斜长角闪岩为主,为角闪岩相的中低级区域变质岩系;主要矿石类型为条带状磁铁石英岩型和块状磁铁角闪岩型。为确定该矿床含矿建造的原岩、变质时代及构造背景,重点对含矿岩系中的斜长角闪岩进行了岩石地球化学和锆石U-Pb年代学研究。结果表明：斜长角闪岩的地球化学特征表现为富集大离子亲石元素、轻微富集重稀土元素;主量元素质量分数与中性-基性岩类基本相似,结合原岩恢复图解,判断其原岩类型为亚碱性玄武岩（拉斑玄武岩）,形成于弧后盆地背景;LA-ICP-MS锆石U-Pb年代学研究中,2个较老的锆石测点年龄分别为（2 468±15）和（2 469±9）Ma,代表区内峰期变质年龄（约2 460 Ma）,26个锆石测点的测年数据较为集中,加权平均年龄为（2 275±25）Ma,代表区内退变质年龄。通过与国内外典型BIF型铁矿床的对比研究认为,区内的鸡南铁矿与官地铁矿同属Algoma型铁矿床。     

略论我国前寒武纪变质作用和地壳演化的某些特征

本文对中国早前寒武纪太古期、早元古期、中晚元古期的变质作用的分布、岩石类型、变质相特点、变质相划分及同位素年龄数据等进行了论述。将中国前寒武纪地壳划分为华北、西北、华南、西南四个变质区、各变质区有自己的不同特点和演化历史。前寒武纪地壳演化是陆壳增长的历史,区域高温和中温变质作用是太古代原始地壳特有的变质作用。     

喜马拉雅地体的泛非-早古生代造山事件年龄记录

喜马拉雅地体是55±10Ma以来印度陆块与欧亚大陆碰撞而形成的增生地体,位于其中的高喜马拉雅与特提斯-喜马拉雅构造单元的变质基底主要由角闪岩相的富铝变质沉积岩和花岗质片麻岩组成。对两类岩石中锆石的SHRIMPU-Pb测年结果表明,除了记录了20Ma以来的构造事件年龄外,主要保存了529-457Ma的变形和变质事件记录,另外还保存了更早期(>835Ma)的年龄信息。根据20Ma以来崛起的喜马拉雅挤出岩片中包含早期强烈褶皱和向南的斜向逆冲构造以及伴随的角闪岩相变质作用记录,结合岩石测年所获得的大量泛非-早古生代年龄和奥陶纪底砾岩的发现,说明曾位于南半球印度陆块北部的变质基底岩石经历过泛非-早古生代造山事件,同位素年代学数据表明:(1)原始喜马拉雅山是泛非-早古生代造山事件的产物;(2)印度陆块早-中元古代变质基底的再活化在原始喜马拉雅山形成中起重要的作用;(3)现在的喜马拉雅山是在泛非-早古生代造山事件基础上再造山的结果。     

P-T conditions of earliest Late Cretaceous metamorphism in the Western Sonobari Complex,northwestern Mexico: tectonic implications

The Western Sonobari Complex in northwestern Mexico consists of metamorphosed rocks mostly derived from Palaeozoic (?) sedimentary and Mesozoic igneous protoliths. Rocks of this complex display amphibolite facies orogenic metamorphism, pervasive foliation, migmatization, and four folding phases. These features are ascribed to a contractional tectonic event with NNW–SSE shortening direction, which caused thrusting, thickening of the crust, and sinking of the lithological units. U–Pb geochronology of migmatitic leucosome bands indicates that peak metamorphic conditions were reached between ~93 and 89 Ma. Post-tectonic Late Cretaceous peraluminous aplite-pegmatite dikes transect the metamorphic foliation. Traditional thermobarometry in the metamorphic rocks yields average pressures and temperatures of 9.0–7.1 kbar and 745–663°C, typical of intermediate P/T Barrovian metamorphism. On the basis of its age and contractional character, the thickening event originating the metamorphism may be related to collision of the Alisitos island arc against crustal blocks of Mexico. Thermobarometric data of post-tectonic intrusives including Late Cretaceous granodiorite and Eocene gabbro indicate emplacement within an overthickened crust, while P-T conditions of post-tectonic dikes point towards an almost isothermal decompression path along the amphibolite facies field. Rock units of similar age and metamorphic character are discontinuously exposed from the Islas Marias offshore the Nayarit coast to the Peninsular Ranges batholith of Baja California, and even extend north into the Sierra Nevada batholith and the Sevier hinterland. This extensive belt of Barrovian metamorphic rocks thus provides a record of middle Cretaceous shortening and crustal thickening related to arc-continent collision followed by subduction resuming.