新疆塔里木北缘色日牙克依拉克一带泥盆纪花岗质侵入体的确定及其地质意义

位于新疆南天山色日牙克依拉克一带发育较多的中酸性侵入岩,前人均划为石炭纪侵入岩,作者对其中花岗闪长岩锆石进行SHRIMP定年,获得侵入岩形成年龄387±8Ma,另有一组捕获锆石的年龄为418.4±6.5Ma,通过对这些侵入岩的岩石学研究,结果显示该区泥盆纪侵入岩均具较典型活动陆缘侵入岩的特征.结合区域岩浆活动资料及前人成果,初步确定在塔里木北缘木札尔特、色日牙克依拉克、虎拉山南缘野云沟至库尔勒一带在古生代为一活动陆缘.     

新疆南天山盲起苏晚石炭世侵入岩的确定及对南天山洋盆闭合时限的限定

南天山额尔宾山中部出露的盲起苏侵入岩主要由花岗闪长岩组成,前人认为是泥盆纪产物,对其中锆石进行SHRIMP定年,获得侵入岩形成年龄296.9±5.4Ma和304.2±11.6Ma。其岩石学具有偏铝和过铝同碰撞花岗岩的特征,结合该带二叠纪后造山花岗岩及其它方面的区域资料,从而提出南天山古生代洋盆的闭合时限为晚石炭世。     

Time scales and length scales in magma ?ow pathways and the origin ofmagmatic Ni-Cu-PGE ore deposits

Ore forming processes involve the redistribution of heat, mass and momentum by a wide range of processes operating at different time and length scales. The fastest process at any given length scale tends to be the dominant control. Applying this principle to the array of physical processes that operate within magma flow pathways leads to some key insights into the origins of magmatic Ni-Cu-PGE sulfide ore deposits. A high proportion of mineralised systems, including those in the super-giant Noril'sk-Talnakh camp, are formed in small conduit intrusions where assimilation of country rock has played a major role. Evidence of this process is reflected in the common association of sulfides with varitextured contaminated host rocks containing xenoliths in varying stages of assimilation. Direct incorporation of S-bearing country rock xenoliths is likely to be the dominant mechanism for generating sulfide liquids in this setting. However, the processes of melting or dissolving these xenoliths is relatively slow compared with magma flow rates and, depending on xenolith lithology and the composition of the carrier magma, slow compared with settling and accumulation rates. Chemical equilibration between sulfide droplets and silicate magma is slower still, as is the process of dissolving sulfide liquid into initially undersaturated silicate magmas. Much of the transport and deposition of sulfide in the carrier magmas may occur while sulfide is still incorporated in the xenoliths, accounting for the common association of magmatic sulfide-matrix ore breccias and contaminated "taxitic" host rocks. Effective upgrading of so-formed sulfide liquids would require repetitive recycling by processes such as reentrainment, back flow or gravity flow operating over the lifetime of the magma transport system as a whole. In contrast to mafic-hosted systems, komatiite-hosted ores only rarely show an association with externally-derived xenoliths, an observation which is partially due to the predominant formation of ores in lava flows rather than deep-seated intrusions, but also to the much shorter timescales of key component systems in hotter, less viscous magmas. Nonetheless, multiple cycles of deposition and entrainment are necessary to account for the metal contents of komatiite-hosted sulfides. More generally, the time and length scale approach introduced here may be of value in understanding other igneous processes as well as non-magmatic mineral systems.     

安徽庐枞盆地南部花岗岩的成岩时代及其意义

庐枞中生代火山盆地位于长江中下游断陷带,地处扬子板块的北缘。庐枞盆地内火山岩和侵入岩分布广泛,包括龙门院、砖桥、双庙和浮山4组火山岩以及34个出露地表的侵入岩体。庐枞盆地内与成矿关系密切的侵入岩体的时空格架和岩浆演化等相关研究工作已经广泛开展,但对于盆地南部的花岗岩类侵入岩研究工作则相对薄弱,制约了对庐枞盆地岩浆作用及深部成矿预测的深入研究。本次工作在详细野外地质工作的基础上,系统对盆地南部的沙墩岩体、船艄石岩体、青山岩体、何家凹岩体、梅林岩体、古塘冲岩体和周家山岩体开展了岩石学、地球化学和年代学研究,确定其主要岩性为石英正长岩和钾长花岗岩,岩石全碱含量均较高,准铝质或过铝质,稀土含量中等,较强Eu亏损,呈略右倾的海鸥型稀土配分模式,成岩时代分别为：126.5±1.1 Ma、126.2±1.2 Ma、126.7±1.3 Ma、126.9±1.4 Ma、125.9±1.3 Ma、126.7±1.4 Ma和124.9±1.3 Ma,均形成于早白垩世。通过与庐枞盆地不同侵入岩的系统对比,提出这些岩体与庐枞盆地晚期侵入岩时代接近,岩石学特征类似,应为同一岩浆作用的产物,形成于区域岩石圈伸展的构造环境。     

Time scales and length scales in magma flow pathways and the origin of magmatic Ni–Cu–PGE ore deposits

Ore forming processes involve the redistribution of heat, mass and momentum by a wide range of processes operating at different time and length scales. The fastest process at any given length scale tends to be the dominant control. Applying this principle to the array of physical processes that operate within magma flow pathways leads to some key insights into the origins of magmatic Ni–Cu–PGE sulfide ore deposits. A high proportion of mineralised systems, including those in the super-giant Noril'sk-Talnakh camp, are formed in small conduit intrusions where assimilation of country rock has played a major role. Evidence of this process is reflected in the common association of sulfides with vari-textured contaminated host rocks containing xenoliths in varying stages of assimilation. Direct incorporation of S-bearing country rock xenoliths is likely to be the dominant mechanism for generating sulfide liquids in this setting. However, the processes of melting or dissolving these xenoliths is relatively slow compared with magma flow rates and, depending on xenolith lithology and the composition of the carrier magma, slow compared with settling and accumulation rates. Chemical equilibration between sulfide droplets and silicate magma is slower still, as is the process of dissolving sulfide liquid into initially undersaturated silicate magmas. Much of the transport and deposition of sulfide in the carrier magmas may occur while sulfide is still incorporated in the xenoliths, accounting for the common association of magmatic sulfide-matrix ore breccias and contaminated “taxitic” host rocks. Effective upgrading of so-formed sulfide liquids would require repetitive recycling by processes such as re-entrainment, back flow or gravity flow operating over the lifetime of the magma transport system as a whole. In contrast to mafic-hosted systems, komatiite-hosted ores only rarely show an association with externally-derived xenoliths, an observation which is partially due to the predominant formation of ores in lava flows rather than deep-seated intrusions, but also to the much shorter timescales of key component systems in hotter, less viscous magmas. Nonetheless, multiple cycles of deposition and entrainment are necessary to account for the metal contents of komatiite-hosted sulfides. More generally, the time and length scale approach introduced here may be of value in understanding other igneous processes as well as non-magmatic mineral systems.     

冀东峪耳崖和牛心山花岗岩体SHRIMP锆石U-Pb定年及其意义

侵入冀东马兰峪复背斜的前寒武岩层中的花岗岩体与区内金矿床有密切的空间关系 ,但成岩时代有争议 ,与金矿床的成因关系不明。本文采用SHRIMP定年方法对含有金矿床的峪耳岩和牛心山岩体进行了锆石U Pb年龄测定 ,获峪耳崖岩体灰白色花岗岩的2 0 6Pb 2 3 8U年龄 175± 1Ma ,肉红色花岗岩的年龄为 174± 3Ma ;牛心山花岗岩的年龄为 172± 2Ma。这 3个年龄在误差范围内可看成是同时代的产物 ,属中生代燕山期。这组年龄限定了金矿化年龄的下限 ,表明金矿化发生在中生代燕山中期后 ,但并不能精确厘定金矿化的年龄 ,因为花岗岩侵入后 ,金矿化之前 ,还有脉岩侵入 ,金矿化与花岗岩本身并无直接的成因关系。     

Ar–Ar ages of intrusions in East Greenland: Rift-to-drift transition over the Iceland hotspot

Sixteen ⁴⁰Ar–³⁹Ar ages are presented for alkaline intrusions to appraise prolonged post-breakup magmatism of the central East Greenland rifted margin, the chronology of rift-to-drift transition, and the asymmetry of magmatic activity in the Northeast Atlantic Igneous Province. The alkaline intrusions mainly crop out in tectonic and magmatic lineaments orthogonal to the rifted margin and occur up to 100 km inland. The area south of the Kangerlussuaq Fjord includes at least four tectonic lineaments and the intrusions are confined to three time windows at 56–54 Ma, 50–47 Ma and 37–35 Ma. In the Kangerlussuaq Fjord, which coincides with a major tectonic lineament possibly the failed arm of a triple junction, the alkaline plutons span from 56 to 40 Ma. To the north and within the continental flood basalt succession, alkaline intrusions of the north–south trending Wiedemann Fjord–Kronborg Gletscher lineament range from 52 to 36 Ma.

We show that post-breakup magmatism of the East Greenland rifted margin can be linked to reconfiguration of spreading ridges in the Northeast Atlantic. Northwards propagation of the proto-Kolbeinsey ridge rifted the Jan Mayen micro-continent away from central East Greenland and resulted in protracted rift-to-drift transition. The intrusions of the Wiedemann Fjord–Kronborg Gletscher lineament are interpreted as a failed continental rift system and the intrusions of the Kangerlussuaq Fjord as off-axis magmatism. The post-breakup intrusions south of Kangerlussuaq Fjord occur landward of the Greenland–Iceland Rise and are explained by mantle melting caused first by the crossing of the central East Greenland rifted margin over the axis of the Iceland mantle plume (50–47 Ma) and later by uplift associated with regional plate-tectonic reorganization (37–35 Ma). The Iceland mantle plume was instrumental in causing protracted rift-to-drift transition and post-breakup tholeiitic and alkaline magmatism on the East Greenland rifted margin, and asymmetry in the magmatic history of the conjugate margins of the central Northeast Atlantic.     

Evidence for multiple Palaeoproterozoic thermal events and magmatism adjacent to the Broken Hill Pb---Zn---Ag orebody, Australia

Thermal events at 1690-1680, 1660-1640 and 1600-1570 Ma have been resolved by SHRIMP U---Pb geochronological study on zircons and monazites from seven localities near to the Broken Hill Pb---Zn---Ag orebody, Australia. The earliest-recognized thermal event included intrusion of now deformed granites such as Rasp Ridge Gneiss and Alma Gneiss and intrusion of gabbro at Round Hill. Previously these have been interpreted as volcanic in origin, and have been assigned to different stratigraphic units of the Palaeoproterozoic Willyama Supergroup. Because these rocks are intrusions, they should be removed from the Supergroup stratigraphic sequence. The 1640–1660 Ma thermal event reached upper amphibolite to granulite conditions and produced melt segregations in parts of the Rasp Ridge Gneiss. Granites of this age are the Purnamoota Road Gneiss, previously correlated with 1690-1680 Ma rocks assigned to the Hores Gneiss stratigraphic unit, and granitic veins within Sundown Group metapelites. The 1600-1570 Ma thermal event also reached upper amphibolite to granulite conditions. The only possible 1600-1570 Ma intrusive rock reported in this study is ‘Lf-leucogneiss’ (granite) at the Purnamoota Road locality. Melt segregations of this age have been found in the Round Hill gabbro and metamorphic segregations have been found in the Purnamoota Road Gneiss. The granite intrusions and segregations are absolute time markers for fabric development and therefore can be used to re-evaluate tectonothermal evolution of rocks close to the Broken Hill Pb---Zn orebody. Within the studied rocks several discrete high grade deformation phases have been observed. The earliest detected deformation is older than 1640–1660 Ma, but syn- or post 1690 Ma. A later deformation phase can be constrained to be pre-or syn 1640–1660 Ma and a yet later deformation phase to be syn- or post- 1600-1570 Ma. The current consensus classifies the Broken Hill Pb---Zn---Ag orebody as the metamorphosed equivalent of classic SEDEX (sedimentary-exhalative) deposits, deposited at ca 1690 Ma. This interpretation heavily relies on the Hores Gneiss being a volcanic marker horizon, because the orebody is situated, apparently conformably, within the Hores Gneiss. However, results of this study show that rocks assigned to the Hores Gneiss are of different age, thus do not form a reliable marker horizon. The present results suggest that in the Thackaringa and Broken Hill Groups in the vicinity of Broken Hill, true supracrustal rocks are ≥ 1690 Ma, rather than ca 1690 Ma as previously suggested. Large parts of rocks surrounding the orebody are intrusions and together with their host supracrustal rocks were metamorphosed and locally remelted at 1660-1640 and 1600-1570 Ma.     

扬子地台北缘檬子地区侵入岩年代格架和岩石成因研究

对扬子地台北缘檬子地区发育的辉长苏长岩、角闪辉长岩、花岗闪长岩、石英闪长岩和钾长花岗岩体进行了LA-ICPMS锆石U-Pb定年和Lu-Hf同位素、全岩主量和微量元素分析。结果表明,该区基性和中酸性岩体均形成于新元古代中晚期。其中基性侵入体辉长苏长岩形成于764±38Ma,角闪辉长岩形成于757±32Ma,均属钙碱性系列,轻稀土富集,微量元素原始地幔蛛网图中显示富集大离子亲石元素(LILE,如Ba、La、Ce、Sr等)、具有明显的Nb、Ta负异常和弱的P、Ti具负异常,形成于地幔原始石榴石二辉橄榄岩源区,部分熔融的程度大于10%。锆石Lu-Hf同位素研究揭示辉长苏长岩ε_Hf(t)值变化于+6.92~+13.99,t_DM变化于736~1058Ma,当ε_Hf(t)为最大正值时,对应的t_DM与岩石的形成年龄基本一致;角闪辉长岩ε_Hf(t)变化于+4.74~+14.97,t_DM主体范围变化于707~1214Ma,当ε_Hf(t)具有最大的正值时,t_DM接近于岩石形成年龄。反映出辉长苏长岩和角闪辉长岩在形成过程中,经历了地壳混染作用,最有可能是中元古代地壳的混染。对石英闪长岩年代学测试表明其形成于774±34Ma。花岗闪长岩、石英闪长岩属钙碱性系列,具有相似的稀土和微量元素分布特点,表现为大离子亲石元素富集,亏损高场强元素(Ta、Nb、Y、Yb等)的特点。钾长花岗岩样品属高钾钙碱系列,岩石稀土元素丰度较低,∑REE=105.9×10^-6~133.1×10^-6,轻稀土富集,具有强烈的Eu负异常(δEu=0.03~0.09),微量元素原始地幔标准化表现为大离子亲石元素富集,Ba、Nb、Sr、P、Ti具有明显负异常。石英闪长岩ε_Hf(t)变化于-4.46~+13.10之间,大部分为正值,平均值为+4.80,t_2DM变化于820~1726Ma,平均值为1256Ma,显示其主要为新元古代新生地壳部分熔融的产物,在形成过程中有中元古代地壳物质的参与;钾长花岗岩ε_Hf(t)主体为负值,变化于-16.49~+8.33,平均值为-5.26,t_2DM为943~2080Ma,平均值为1458Ma,显示其主体为古老地壳物质部分熔融的产物,新元古代新生地壳参与较少。结合区域地质特征分析,檬子地区基性和中酸性侵入体形成于活动大陆边缘弧环境,是新元古代勉略洋盆向南俯冲的地质记录。     

姚安和马厂箐富碱侵入岩体的地球化学特征

姚安岩体和马厂箐岩体属哀牢山-金沙江富碱侵入岩带的组成部分,位于NW-WNW向哀牢山-金沙江深大断裂东侧。姚安岩体主要为正长斑岩,马厂箐岩体主要为花岗斑岩两岩体化学成分上具有富碱、高钾的特点。岩石富轻稀土和Rb、Sr、Ba、Th、La等大离子亲石元素而亏损Nb、Ta和Ti等高场强元素。岩石的Rb/Sr、La/Nb和Ba/Nb以及LREE/HREE比值高,铕异常不明显,ISr值偏高,εNd值偏低。研究表明:(1)姚安岩体和马厂箐岩体形成于喜山期大陆弧伸展环境。两岩体的岩浆均起源于富集地幔(EMII),其壳幔混合特征,主要是俯冲带地幔楔内壳-幔物质源区混合的结果;(2)尽管姚安和马厂箐岩体在成岩构造背景、成岩时代、成岩方式和源区特征以及成分上高碱富钾等方面具相似性,但它们在岩性和组成上仍存在较大差异。这种差异可能是由源区物质部分熔融程度不同,以及岩浆结晶时温压和氧逸度等条件的不同所造成;(3)金矿化主要与相对高温、低压和高氧逸度的碱性系列岩浆活动有关,而铜矿化则主要与相对低温、高压和低氧逸度的碱性-亚碱性过度系列岩浆活动有关。