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151.
Victorian granites containing more than 750 ppm Ba are almost entirely confined to the region between a line from Geelong to Swan Hill and the Wonnangatta Fault Zone. Granite Ba contents normalised to 70% SiO2 range from 620 to 733 ppm in western Victoria, 719 to 1560 ppm in central Victoria and 493 to 689 ppm in eastern Victoria. Melting of Ba-rich (meta)sedimentary rocks in the lower – middle crust is implicated in the petrogenesis of central Victorian S types, at least. Thus, granite geochemistry supports the concept of some sort of (largely concealed) Ba-rich Selwyn Block beneath central Victoria, although the boundaries that have been proposed for the block are modified here. There is a strong possibility that the Selwyn Block is an exotic terrane emplaced by northwest-directed strike-slip movement during the Bindian Orogeny. Such movement appears to have been controlled by the previously postulated Baragwanath Transform and another fundamental fault here called the Ulrich Transform. Asthenospheric upwelling related to movement on the Ulrich and Baragwanath Transforms may be the explanation for the twin belts of 400 Ma plutonism occurring to the west of the former and to the east of the latter. The southern extension of the Ulrich Transform may be the Tamar – Tiers structure in Tasmania. Plate-tectonic models suggesting Ordovician – Silurian subduction in Victoria need to be carefully revisited given the possibility of Siluro-Devonian exotic terrane emplacement. 相似文献
152.
R. T. Pidgeon 《Australian Journal of Earth Sciences》2013,60(2):237-242
U‐Pb isotopic systems of zircons from the Boobina and Spinaway Porphyries from the Precambrian Pilbara Block of Western Australia indicate ages of 3307± 19 Ma and 2768 ± 16 Ma, respectively. The Boobina Porphyry intrudes upper members of the Archaean greenstones of the Warrawoona Group. The Spinaway Porphyry intrudes basal units of the unconformably overlying volcanics and sediments of the Mt Bruce Supergroup. The age of the Boobina Porphyry, together with previous zircon U‐Pb and whole rock Sm‐Nd age determinations on stratigraphically older units, indicate that early Archaean volcanism in the Pilbara took place between 3560 Ma and 3300 Ma. On the basis of the age determination of the Spinaway Porphyry, and the chronometric definition of 2500 Ma for the Archaean—Proterozoic boundary, by the International Subcommis‐sion on Precambrian Stratigraphy (James H. L. 1978, Precambrian Res. 7, 193–204), the lower units of the Mt Bruce Supergroup should now be assigned to the Archaean. 相似文献
153.
The Parnell Quartz Monzonite in the Pilbara Block of Western Australia is a Proterozoic (1731 ± 14 Ma) pluton characterized by high modal K‐feldspar and a greater abundance of hornblende relative to biotite, as is typical of Phanerozoic monzonitic rocks in eastern Australia. The only geochemical features reflecting its setting in an Archaean terrain are high Na2O, Ni and Cr. The pluton is zoned, with an increase in K‐feldspar, quartz and biotite and a decrease in plagioclase and hornblende from margin to core. Chemically, this zoning is reflected by systematic variation of CaO, K2O, Na2O, Sr and Rb, but ferromagnesian elements have irregular trends, implying preferential extraction of feldspars relative to mafic minerals during differentiation of the magma. The unusual geochemical trends are explained by a model involving ‘in situ’ feldspar fractionation of a K‐rich residual liquid from a mafic crystalline mush. A parent magma similar to the average rock composition of the pluton is deduced because high ferromagnesian trace element abundances preclude extensive fractionation of mafic minerals. Geochemical and isotopic constraints suggest that the ultimate source was chemically similar to a shoshonitic basaltic andesite, that must have been emplaced beneath the eastern margin of the Pilbara Block in the Early Proterozoic. Subsequent partial melting of this postulated underplated source at ~ 1700 Ma to produce the Parnell Quartz Monzonite was probably associated with tectonism in the Gregory Range Complex. 相似文献
154.
The provenance of the large and super-large scale bauxite deposits developed in the Wuchuan–Zheng’an–Daozhen (WZD) alumina metallogenic province in the Yangtze Block of South China is poorly understood. LA-ICP-MS and SIMS U–Pb dating of detrital zircons from bauxite ores and the underlying Hanjiadian Group in the WZD area provide new constrains on the provenance of the WZD bauxite and provide new insight on the bauxite ore-forming process. The ages of the detrital zircons in the bauxites and the zircons in the Hanjiadian Group are similar suggesting that the bauxites are genetically related to the Hanjiadian sediments. The detrital zircon populations of the four samples studied show four primary age peaks: 2600–2400 Ma, 1900–1700 Ma, 1300–700 Ma and 700–400 Ma. The age distribution of detrital zircons indicates that they are probably derived from various sources including Neoproterozoic, Mesoproterozoic, Paleoproterozoic, Archean and some minor Paleozoic sources. The most abundant age population contains a continuous range of ages from 1300 to 700 Ma, ages consistent with subduction-related magmatic activities (1000–740 Ma) along the western margin of the Yangtze Block and the worldwide Grenville orogenic events (1300–1000 Ma). Thus, it is suggested that the main provenances of the WZD bauxite and the Hanjiadian Group are the Neoproterozoic igneous rocks in the western Yangtze Block and the Grenville-age igneous rocks in the southern Cathaysia Block. In addition, this work verifies that the global Grenville orogenic events and subduction-related magmatic activities associated with the Yangtze Block had a significant influence on the formation of the WZD bauxite deposits. 相似文献
155.
华北深部岩石圈存在弱的新元古代热活动的同位素年代学信息:证据及意义 总被引:3,自引:2,他引:1
人们普遍认为华北区别于华南的主要特征在于缺少广泛的新元古代岩浆活动,但原因是什么还不清楚。本文汇总了华北四个地区深源岩石包体中有这样同位素年龄的结果,并就它们所反映的华北当时在Rodinia超大陆裂解中心的位置和可能的岩石圈厚度进行了讨论。这些深源岩石包体分别是辽宁复县古生代金伯利岩中的基性麻粒岩、河北涉县碳酸岩化金伯利岩中石榴石辉石岩、河南信阳中生代火山岩中的橄榄岩及河北汉诺坝新生代玄武岩中橄榄岩。所记录的新元古代年龄信息包括:复县基性麻粒岩锆石0.61Ga的下交点年龄,涉县石榴石辉石岩全岩-单矿物的0.76GaSm-Nd内部等时年龄、信阳橄榄岩锆石的新元古代(>0.64Ga)年龄以及汉诺坝橄榄岩硫化物0.9~0.6Ga的Re-Os年龄。与华南广泛发育新元古代岩浆活动不同,华北有限的新元古代年龄信息(包括地表出现的)可能反映着它们当时在Rodinia超大陆的位置有所不同,即华南更靠近于超大陆裂解的中心、而华北可能远离该中心(如边缘)。位置的差别也预示着当时华南岩石圈的可能比较薄、而华北具巨厚的岩石圈。因此,我们认为弱的热事件和巨厚的岩石圈可能是造成华北新元古代热活动不明显的原因。 相似文献
156.
阿拉善高原处于季风边缘区,生态环境极其脆弱,对气候干湿变化的响应极为敏感。利用近60a的逐月气温和降水量资料,采用非参数Mann-Kendall和Mann-whitney阶段性转换检验方法,分析了其变化趋势。结果表明,研究区气温在1988年前后发生突变,在该跃点后的19882010年的平均气温比19551987年要高1℃以上,有明显的增温趋势。而年降水量则无明显的增加趋势,尽管部分站点在1991年前后有微弱的增加迹象,但是区域尺度的降水突变还不存在,气候仍以暖干化为主,暂不能得出气候向暖湿转变的结论。 相似文献
157.
根据野外观察、测量与分析,特别是综合华北地块南部断裂体系第四纪活动性质的构造和地貌标志,表明现今华北地块南部NWWNW向断裂活动最为显著,主要表现为左旋走滑性质;在前新生代构造基础上发育的三门峡-鲁山-舞阳断裂带和新构造期发育的新乡-商丘断裂带是具有走滑性质的新生代壳内活动断裂。地球物理资料表明,在介休-新乡-溧阳和巴东-泉州-台湾地震带西北部的深部存在两个NW向构造带,在地幔可能汇聚为一条构造带。综合这些断裂及其所控断陷盆地的展布特征,明确了该区的NE向、NW向及近EW向断裂的运动学关系。即在应力应变基底格局的制约下,两个NW向构造带强烈的左旋走滑拉分运动作用下导致华北地块南部发育拉分盆地,NW向新断裂的形成和先存NNE、NW及近EW向断裂的复活,控制了新生代复杂的断裂或断块构造格局的形成。 相似文献
158.
通过1:250 000衢州区域地质调查,在武夷地块浙西南地区发现古元古代镁铁质-超镁铁质岩石。该套岩石在金华、龙游-带呈面状分布,金华张村出露最为完整,主要由辉石角闪石岩、角闪石岩、斜长阳起石岩、斜长辉石岩和斜长角闪岩等组成。对辉石角闪石岩与阳起石岩开展了LA-ICP-MS锆石U-Pb测年分析,分别获得其成岩年龄为1 834±14 Ma(MSWD=0.23,N=16)、1 839±17 Ma(MSWD=0.22,N=11),暗示这套镁铁质-超镁铁质岩石为古元古代岩浆活动的产物。这一发现表明,约1 830 Ma武夷地块处于板内伸展构造环境,同时该时期武夷地块已经具备足够的刚性,以致产生大规模的脆性破裂以及可能由地幔对流作用引发的基性岩浆活动,推测在约1.83 Ga武夷地块已经具有克拉通的性质。 相似文献
159.
160.
气候变化对阿拉善荒漠植被的影响研究 总被引:15,自引:10,他引:5
利用1982—2003年NASA GIMMS归一化植被指数(NDVI)数据集和阿拉善左旗、右旗和额济纳旗气温和降水资料,对阿拉善地区NDVI变化和气候变化特征及其相互关系进行了分析。结果表明:①1982—2003年东部地区(左旗植被指数略有增加,而中部(右旗)和西部地区(额济纳旗)则呈下降趋势。季节变化东、中、西部表现不同。左旗和额济纳旗多年平均NDVI的月变化和右旗相反。②东部地区升温率最大,其次是西部和中部地区。降水表现为东部和西部地区(额济纳旗)稍有增加,而中部地区(右旗)呈缓慢减少趋势。季节变化东、中、西部表现不同。多年平均气温和降水量的月变化趋势东、中、西部一致,呈单峰型曲线。③东部地区和中部地区,降水量与植被指数存在明显的年相关、隔季和当季相关,而西部地区额济纳旗没有明显的相关性。阿拉善地区NDVI同温度的相关性不好。 相似文献