The Orogeny in the Altaides1

Abstract Studies of the deformation styles, formation types and isotopic age data indicate that the Altaides has successively experienced 5 stages of orogeny: (1) the Kanas orogeny forming the angular unconformity between the Baihaba Formation (O₃) and the Habahe Group (Z-O₂); (2) the Daqiao orogeny (S₃-D_1-2) giving rise to the early Hercynian quasi-aulacogen extensional continental crust of the area; (3) the Altay orogeny (middle- late Hercynian) leading to the oblique intracontinental collision and the formation of large shear arc-shaped thrust system and representing a strong orogeny stage; (4) the pan-Altay orogeny (latest Hercynian-Indosinian) resulting in the uplifting and erosion of the mountains as a whole; (5) the Himalayan movement causing the rejuvenation of fault systems and block uplift of the Altaides since the Cenozoic.     

大别山印支运动的同位素年代学证据

本文根据Sm-Nd同位素年代学成果论证了大别山印支运动的表现。主要依据:(1)阿尔卑斯型橄榄岩的侵入(244±11Ma);(2)辉石岩类超基性岩浆岩的侵入(231±61Ma);(3)高压低温变质岩(如C型榴辉岩)的生成(221±20-224±5Ma)。它们都发生在印支早期,并从而证明了华北陆块、大别山微型陆块、扬子陆块三者是在早三叠世拼接在一起的。在此之前,华北陆块和扬子陆块之间存在一古特提斯洋。     

Paleoproterozoic granitoids from the northern limit of the Archean Amapá block (Brazil), southeastern Guyana Shield: Pb–Pb evaporation in zircons and Sm–Nd geochronology

Central Amapá, northern Brazil is located at the boundary between: (a) a northern Paleoproterozoic domain, consisting mainly of granite-greenstones terrains and (b) a southern Archean continental block (Amapá block), including an Archean basement reworked during the Transamazonian orogeny (2.26–1.95 Ga). Field investigations, Pb–Pb zircon and Sm–Nd whole rock geochronology supported by geochemical data on granitoids brought further constraints on Paleoproterozoic crustal growth in the southeastern Guyana Shield. A first magmatic episode, dated at 2.26 Ga, is marked by the crystallization of metaluminous low-K tholeiitic tonalites and quartz-diorites, which geochemical affinity with volcanic arc and association with T-MORB amphibolites suggest that they formed in a back-arc basin – island arc system. This event is coeval to the oceanic stage registered in French Guyana during the Eorhyacian (2.26–2.02 Ga). A second magmatic episode is represented by peraluminous, medium- to high-K calc-alkaline tonalite and granodiorite, which revealed some similarities with Mesorhyacian TTG rocks of French Guyana. For granitoids of both episodes, T_DM and ε_Nd values indicate the contribution of some Archean crustal component, probably by assimilation or contamination. This second magmatic episode occurred at 2.10 Ga, indicating that the period of successive calc-alkaline magmatic arcs formation may have extended until the Neorhyacian. Meanwhile, during this time, tectonic accretion by collision of the newly formed continental landmass was the prevailing process in French Guyana. The latter magmatic episode, even though poorly constrained, was registered around 2.08–2.02 Ga in central Amapá. It corresponds to the emplacement and solidification of high-K collisional granitoids, produced by partial melting of the Archean continental crust, as testified by the Archean T_DM, inherited Pb–Pb zircon ages and strongly negative ε_Nd values. Our results point toward the existence of a protracted episode of crustal growth during the Neorhyacian in the southeastern Guyana Shield. This episode has been predominantly driven by magmatic arc accretion during, at least, 160 My, along the period of 2.26–2.10 Ga. This cycle ended with diachronic closure of the oceanic basins and arc–continent collision.     

The effects of Dissolution–Precipitation Creep on quartz fabrics within the Purgatory Conglomerate,Rhode Island

Quartz Crystallographic Preferred Orientation (CPO) patterns are most commonly a result of deformation by dislocation creep. We investigated whether Dissolution–Precipitation Creep (DPC), a process that occurs at lower differential stresses and temperatures, may result in CPO in quartz. The Purgatory Conglomerate is part of the SE Narragansett basin where strain intensity increases from west to east and is associated with top-to-the-west transport and folding during the Alleghanian orogeny. Within the Purgatory Conglomerate, DPC led to quartz dissolution along cobble surfaces perpendicular to the shortening direction, and quartz precipitation in overgrowths at the ends of the cobbles (strain shadows), parallel to the maximum extension direction. Quartz c-axis orientations as revealed by Electron Backscatter Diffraction (EBSD) methods were random in all analyzed domains within the cobbles and strain shadows irrespective of the intensity of strain or metamorphic grade of the sample. Quartz dissolution probably occurred exclusively along the cobbles' margins, leaving the remaining grains unaffected by DPC. The fact that quartz precipitated in random orientations may indicate that the strain shadows were regions of little or no differential stress.     

墨西哥北部拉拉米期斑岩型铜矿床时空分布及典型矿床特征

北美斑岩型铜矿集区“大集群”是世界重要铜成矿省之一;墨西哥北部索诺拉州斑岩型铜矿带,是其在墨西哥的延伸。斑岩型铜矿床主要沿着拉拉米岩浆弧展布。拉拉米造山作用期间(80～40 Ma),由于法拉隆板块向北美板块俯冲角度的减小,岩浆活动中心逐步向东移动,形成76～70 Ma, 63～57 Ma和46～40 Ma三期岩浆活动高峰期,其中63～57 Ma岩浆活动规模最大。通过对区内已有矿床的成矿时代与岩浆活动时限及矿床分布集中程度的对比研究发现,斑岩型铜成矿高峰期主要集中在63～56 Ma期间。通过典型矿床的成矿特征梳理及区域岩浆作用综合分析,笔者等认为拉拉米造山作用是区内形成巨量铜钼富集的主要地质作用。     

内蒙古东北部晚二叠世-早三叠世构造演化的新证据：尖子山早三叠世磨拉石的识别

磨拉石是造山带的重要组成部分,利用磨拉石来限定碰撞作用时限是造山带研究的一项重要内容。以内蒙古东北部科右中旗尖子山出露的一套杂色砾岩夹紫红色-灰紫色砂砾岩地层为研究对象,通过对其岩石组合、沉积构造、沉积时代的研究,认为其是一套产于陆内造山背景下前陆盆地的近源磨拉石,地层底部的近源崩塌堆积成因的花岗岩巨砾锆石LA-ICP-MS U-Pb年龄为250±2.9 Ma。结合区域上老龙头组碎屑锆石年龄及三叠纪同碰撞-后碰撞-陆内伸展阶段花岗岩时代的研究,认为该套磨拉石沉积于早三叠世,属下三叠统老龙头组。研究区早三叠世磨拉石的识别,明确了早三叠世存在陆内造山作用,为内蒙古东北部晚二叠世-早三叠世构造演化提供了新证据。     

Advances in SHRIMP geochronology and their impact on understanding the tectonic and metallogenic evolution of southern Brazil

Significant improvements, both in understanding the evolution of zircons and in understanding the geotectonic and metallogenetic evolution of the complex terrain of southern Brazil, are obtained from a SHRIMP geochronology study and reviewed in this paper. The use of backscattered electron and cathodoluminescence images, prior to SHRIMP isotopic determinations, proved of enormous fundamental value for this technique. Zircon is a domainal open‐system mineral in many geological conditions; very old domains may be preserved, but the same crystal may show ages of younger tectonic events. Zircons may recrystallise inwards from the rims or outwards from the cores, and also along euhedral high‐U or metamict thin zones. Zircons also may be recrystallised during gold‐related hydrothermalism, phyllic alteration of granitic rocks. The precise dating of amphibolite dykes can be achieved by the identification and dating of magmatic zircons. Precambrian orogenies are identified along with the intervening intracratonic tectonic cycles of supercontinents in southern Brazil from 3300 to 470 Ma. Granulite protoliths were formed during the Jequié Orogeny (ca 2600 Ma), but extensive arc accretion occurred in the Palaeoproterozoic (ca 2250 Ma) Encantadas Orogeny. Late in the Transamazonian Cycle, granites were formed by crustal melting at about 2000 Ma in the Camboriú Orogeny. Both accretionary and collisional orogenies are also identified in the Neoproterozoic Brasiliano Cycle. These are the accretionary Passinho Orogeny (ca 900 Ma) and São Gabriel Orogeny (ca 700 Ma), that were succeeded by the collisional Dom Feliciano Orogeny (ca 600 Ma). Base‐metal and gold deposition occurred in juvenile island arcs and in late orogenic porphyry‐copper‐type magmatic‐hydrothermal settings during the Neoproterozoic.     

Cold subduction and the formation of lawsonite eclogite – constraints from prograde evolution of eclogitized pillow lava from Corsica

A new discovery of lawsonite eclogite is presented from the Lancône glaucophanites within the Schistes Lustrés nappe at Défilé du Lancône in Alpine Corsica. The fine‐grained eclogitized pillow lava and inter‐pillow matrix are extremely fresh, showing very little evidence of retrograde alteration. Peak assemblages in both the massive pillows and weakly foliated inter‐pillow matrix consist of zoned idiomorphic Mg‐poor (<0.8 wt% MgO) garnet + omphacite + lawsonite + chlorite + titanite. A local overprint by the lower grade assemblage glaucophane + albite with partial resorption of omphacite and garnet is locally observed. Garnet porphyroblasts in the massive pillows are Mn rich, and show a regular prograde growth‐type zoning with a Mn‐rich core. In the inter‐pillow matrix garnet is less manganiferous, and shows a mutual variation in Ca and Fe with Fe enrichment toward the rim. Some garnet from this rock type shows complex zoning patterns indicating a coalescence of several smaller crystallites. Matrix omphacite in both rock types is zoned with a rimward increase in X_Jd, locally with cores of relict augite. Numerous inclusions of clinopyroxene, lawsonite, chlorite and titanite are encapsulated within garnet in both rock types, and albite, quartz and hornblende are also found included in garnet from the inter‐pillow matrix. Inclusions of clinopyroxene commonly have augitic cores and omphacitic rims. The inter‐pillow matrix contains cross‐cutting omphacite‐rich veinlets with zoned omphacite, Si‐rich phengite (Si = 3.54 apfu), ferroglaucophane, actinolite and hematite. These veinlets are seen fracturing idiomorphic garnet, apparently without any secondary effects. Pseudosections of matrix compositions for the massive pillows, the inter‐pillow matrix and the cross‐cutting veinlets indicate similar P–T conditions with maximum pressures of 1.9–2.6 GPa at temperatures of 335–420 °C. The inclusion suite found in garnet from the inter‐pillow matrix apparently formed at pressures below 0.6–0.7 GPa. Retrogression during initial decompression of the studied rocks is only very local. Late veinlets of albite + glaucophane, without breakdown of lawsonite, indicate that the rocks remained in a cold environment during exhumation, resulting in a hairpin‐shaped P–T path.     

河南省金矿构造控矿研究

本文运用地洼成矿理论对河南省内金矿床的构造控矿特征进行了总结 ,认为构造是主要的控矿因素 ,它们主要表现为不同型式、不同规模、不同期次的构造对金矿成矿的控制作用。并对构造控矿机制进行了探讨。     

大陆岩石圈与软流圈之间的耦合关系—大陆动力学研究的突破口

板块构造学说解释了全球构造的许多现象,但尚未满意地解释大陆构造中的一些问题,例如陆内造山带的形成、造山带和盆地形成的复杂历史和旋回性问题、大陆构造圈的不均一性等。新近提出的一些构造模式,都不约而同地指出了软流圈在大陆构造发展中的重要作用,但尚未成功解决软流圈运动的驱动力问题。本文认为,大陆岩石圈与软流圈之间相互作用关系的深入研究,是阐明大陆动力学过程的关键之一。此外,本文还对“软流体振荡作用”工作假说作了初步讨论。