Late Cambrian to Early Silurian Granitic Rocks of the Gemuri Area, Central Qiangtang, North Tibet: New Constraints on the Tectonic Evolution of the Northern Margin of Gondwana

The Paleozoic tectonic framework and paleo–plate configuration of the northern margin of Gondwana remain controversial. The South Qiangtang terrane is located along the northern margin of Gondwana and records key processes in the formation and evolution of this supercontinent. Here, we present new field, petrological, zircon U-Pb geochronological, and Lu-Hf isotopic data for granitic rocks of the Gemuri pluton, all of which provide new insights into the evolution of the northern margin of Gondwana. Zircon U-Pb dating of the Gemuri pluton yielded three concordant ages of 488.5 ± 2.1, 479.9 ± 8.9, and 438.5 ± 3.5 Ma. Combining these ages with the results of previous research indicates that the South Qiangtang terrane records two magmatic episodes at 502–471 and 453–439 Ma. These two episodes are associated with enriched zircon Hf isotopic compositions(εHf(t) =-10.1 to-3.9 and-16.6 to-6.5, respectively), suggesting the granites were formed by the partial melting of Paleoproterozoic–Mesoproterozoic metasedimentary rocks(Two–stage Hf model ages(TCDM) = 2094–1704 and 2466–1827 Ma, respectively). Combining these data with the presence of linearly distributed, contemporaneous Paleozoic igneous rocks along the northern margin of Gondwana, we suggest that all of these rocks were formed in an active continental margin setting. This manifests that the two magmatic episodes within the Gemuri area were associated with southward subduction in the Proto-(Paleo-) Tethys Ocean.     

The Byers Basin: Jurassic-Cretaceous tectonic and depositional evolution of the forearc deposits of the South Shetland Islands and its implications for the northern Antarctic Peninsula

ABSTRACT

This paper addresses the Jurassic–Cretaceous stratigraphic evolution of fore-arc deposits exposed along the west coast of the northern Antarctic Peninsula. In the South Shetland Islands, Upper Jurassic deep-marine sediments are uncomformably overlain by a Lower Cretaceous volcaniclastic sequence that crops out on Livingston, Snow and Low islands. U-Pb zircon ages are presented for the upper Anchorage Formation (153.1 ± 1.7 Ma) and the Cape Wallace granodiorite of Low Island (137.1 ± 1.7 Ma) as well as ⁴⁰Ar/³⁹Ar ages of 136–139 Ma for Low Island andesites. Data are also presented for a U-Pb age of 109.0 ± 1.4 Ma for the upper volcanic succession of Snow Island. In combination with published stratigraphy, these data provide a refined chrono- and litho-stratigraphic framework for the deposits herein referred to as the Byers Basin. Tentative correlation is explored with previously described deposits on Adelaide and Alexander islands, which could suggest further continuation of the Byers Basin towards the south. We also discuss possible correlation of the Byers Basin with the Larsen Basin, a sequence that shows the evolution of foreland to back-arc deposits more or less contemporaneously with the fore-arc to intra-arc evolution of the Byers Basin.     

Study on the fractal character of magnetic anomaly fields and the delineation of tectonic elements in the South China Sea

INTRODUCTION~hon0ffornjcelmtSwnthebasisofthestudyonthecrustaldefonnationanddeVel0prnent,thedistributionofndnereIs,andthendnerogeneticlawr.ThegeornagneticfieklisthecomP0siterefledionoftheformati0nanddefortnationofthecrust.Sornagnct-icsLLrVeydatacomPriseirnPortantbasisfordelincatingthearchiteCtonicelements,dCtendningtheirboundarylines,andanalopngthesanndarystrudriineachelernent,whicharetthernaintasks0fgoophySicalworkers.Thegoornagneticdelineati0nofanareahadforlongbodonequalitativeybyobs…     

辽东半岛南部早古生代馒头组—冶里组沉积盆地演化

辽东半岛南部早古生代地层发育,其中馒头组—冶里组的沉积过程,具有完整的陆源碎屑岩—碳酸盐岩堆积特点。从层序划分、沉积环境演化、古气候变迁等方面分析,馒头组—冶里组反映了一个完整的碳酸盐岩台地的发展过程,可划分出准备阶段（形成碎屑岩垫板）、生长—发育阶段（形成碳酸盐岩缓坡）及衰亡阶段（发育白云岩盖层）。     

华机构造单元划分及其特征

华南（研究区）由杨子地块东南部和华夏地块组成。前者划分为三个次级构造单元;江南构造城,怀玉山—浙西北构造域,彭庐构造域;后者划分为二个次级构造单元;闽北—浙东南构造域,桂粤湘赣闽构造域。华夏地块相对于扬子地块是比较活动的构造单元。     

华南震旦纪基性火山岩的地球化学及构造环境

华南地区产于震旦纪地层中的火山岩主要见于震旦纪裂谷盆地两侧的边缘,湖南新化和广东高思火山岩为其中的二个发育地区,镜下鉴定为玄武岩系。在ＳｉＯ２－Ｋ２Ｏ＋Ｎａ２Ｏ的分类图中主要为玄武岩类,有一样品为碧玄岩类,含有较多石英杏仁体玻璃质火山岩角砾的样品为玄武质安山岩类。在ＦｅＯ＊－Ｎａ２Ｏ＋Ｋ２Ｏ－ＭｇＯ三角图中所有样品均为拉斑质岩系。样品的重元素丰度模式具大陆－初始裂谷－Ｅ－ＭＯＲＢ拉斑玄武岩的过渡特征,在Ｎｂ、Ｚｒ、Ｙ、Ｔｉ图上属板内环境。在ＴｉＯ２－Ｋ２Ｏ－Ｐ２Ｏ５三角图上显示以大洋环境为主、大陆环境为次。湖南新化拉斑玄武岩的Ｓｍ、Ｎｄ同位素特征反映岩浆来源于亏损的地幔,并受到了老基底的污染。     

New Early Cretaceous palaeomagnetic pole from Córdoba Province (Argentina): revision of previous studies and implications for the South American database

A continental sequence of red beds and interbedded basaltic layers crops out in the Sierra Chica of Córdoba Province, Argentina (31.5°S, 64.4°W). This succession was deposited in a half-graben basin during the Early Cretaceous. We have carried out a palaeomagnetic survey on outcrops of this basin (147 sites in seven localities). From an analysis of IRM acquisition curves and detailed demagnetization behaviour, three different magnetic components are identified in the volcanic rocks: components A, B and X are carried by single- or pseudo-single-domain (titano) magnetite, haematite and multidomain magnetite, respectively. Component A is interpreted as a primary component of magnetization because it passes conglomerate, contact, tilt and reversal tests. The carrier of the primary magnetization, fine-grained (titano)magnetite, is present in basalts with a high degree of deuteric oxidation. This kind of oxidation is interpreted to have occurred during cooling. Components B and X are discarded because they are interpreted as recent magnetizations. In the sedimentary rocks, haematite and magnetite are identified as the carriers of remanence. Both minerals carry the same component, which passes a reversal test. The calculated palaeomagnetic pole, based on 55 sites, is Lat. 86.0°S, Long. 75.9°E ( A ₉₅=3.3, K =35). This palaeomagnetic pole supersedes four with anomalous positions reported in previous papers.     

南北地震带地震活动分析

在综合分析有关南北地震带研究成果的基础上重点分析了南北地震带活动特征;根据该带地震活动特点和规律提出了南北地震带目前处于新的活动期;预测了今后发生强震的潜在危险区（段）为南北地震带的中北段。     

巴士系断裂带的构造应力场特征

利用１２０次地震震源机制解资料,分析了巴士系断裂带构造应力场的基本特征。结果表明,该断裂带应力场的总体特征为ＮＷ￣ＮＷＷ向,但不同地段略有差异。由北西至南东主压应力Ｐ轴方向由ＮＷＷ￣ＮＷ￣ＮＷＷ（近ＥＷ）向,Ｐ轴相对平缓,地震以走错动为主,兼具倾滑分量,应力场的变化与菲律宾海板块对欧亚板块的不均匀推挤作用有关。     

华南晚古生代礁硅岩套的组成和分类及地质涵义

礁硅岩套是一种重要的含矿岩石组合,它是由生物礁相碳酸盐岩、热水沉积岩（矿）、细碎屑岩和泥灰岩、同怪位或相邻地区地层中的火山质岩或潜火山岩,以较为固定的时空有序的状态组合而成。根据生物礁的形成环境和热水沉积作用的产出方式,礁硅岩套可分为盆地型、台地型和过渡型三类。它是在特征地质背景下,在盆地演化的一定阶段内,由盆地正常沉积作物、盆地中的生物作用、盆下源的热水沉积作用和岩石圈深部岩浆作用综合形成的产物