Provenance and tectonic setting of the Jurassic Huayacocotla Formation and Alamitos Sandstone,Central Mexico

We present a modified model for the paleogeographic evolution of Mexico during Early and Late Jurassic time that is constrained by the tectonic setting and the weathering conditions of the Early Jurassic Huayacocotla Formation and Late Jurassic Alamitos Sandstone basins in state San Luis Potosí in central Mexico. Framework petrography constrains feldspato-quartzose sandstone (mean of Q₆₈F₂₂L₁₀) and litho-quartzose (mean of Q₇₅F₆L₁₉) sandstone compositions for the two units, respectively. The abundant lithic fragments are totally dominated by volcanic fragments. Quartz cathodoluminescence colours and textures from the Alamitos Sandstone supports a large input of volcanic material, but also indicates the presence of metamorphic quartz. Similarly, the geochemical composition is more mafic for the Huayacocotla Formation (Th/Sc: ˜0.6 and Cr/Th: ˜10) than for the Alamitos Sandstone (Th/Sc: ˜1.1 and Cr/Th: ˜48). Also the weathering conditions were less intense during the Early (CIA: ˜60, PIA: ˜61) than the Late Jurassic (CIA ˜85, PIA ˜97). Well preserved lithic fragments and feldspar grains, particularly in the Huayacocotla Formation, indicate that weathering indeed was minor for this unit. We interpret the difference between the two units as a combined result of climate change and tectonic setting. During the Early Jurassic, transport of volcanic detritus probably dominated from the active Nazas arc in the west. Later, additional sources from the metamorphic basement of Mexico were included. During Late Jurassic time strike-slip faulting related to the opening of the Gulf of Mexico may have re-directed the sediment-transport systems. Finally, the degree of weathering was affected by drastic climatic change from arid to humid tropical conditions during the Middle to Upper Jurassic, possibly related to the first incursions of Gulf of Mexico marine environments linked to the rotation of the Yucatan block.     

Detrital record of the denudation of volcanic islands under sub-tropical climate (Cape Verde)

This research is focused on the composition of the sediments produced in volcanic islands when the climate does not favour weathering. The XRD mineralogy (bulk sample and fraction finer than 63 μm), petrography and geochemistry of a set of bedload stream and beach samples collected in the “old” Maio and the “young” Fogo islands of Cape Verde archipelago are used to investigate the compositional transformations promoted by exogenous processes during island denudation. The main factor responsible for the variability in sediment composition is the incorporation of biogenic material derived from the evolving shelves; it largely exceeds the effects of the exhumation of different volcanic and basement units. Given the arid climate (and steep land surface in Fogo), only the most labile components of basaltic rocks, such as volcanic glass, are decomposed. The incipient weathering and sorting processes are responsible for the depletion of Al in bedload deposits. The same happens with other elements usually regarded as non-mobile (namely, Nb, Th, REE, etc.), while Mg is concentrated. Thus, weathering indices grounded on the premise that “mobile” elements are lost and “non-mobile” elements are enriched via weathering are useless in Cape Verde bedload sediments. With time, weathering is able to promote Na leaching and the formation of secondary minerals, which tend to retain non-mobile elements released in the earlier stages of alteration (e.g., LREE, Th, Y, Nb, Ta etc.). Sorting processes are responsible for the selective removal of less-dense grains, explaining local differences between beach and stream deposits. Beach placers are enriched in augitic clinopyroxene (occasionally also in olivine in the Fogo island), and Sc, Cr and Co. Niobium and Ta must be hosted in fine-grained particles that are easily windblown and their abundances in dusts may reveal Cape Verde as a source of airborne material crossing the Atlantic Ocean.     

青海东昆仑拉浪麦钨多金属矿区英云闪长岩锆石U-Pb测年、岩石地球化学及找矿意义

拉浪麦钨多金属矿位于青海东昆仑东段察汗乌苏河地区,是近年新发现的矽卡岩型矿床。本文对矿区与成矿关系密切的英云闪长岩进行了详细的岩相学、年代学和地球化学研究,为东昆仑原特提斯岩浆构造演化提供新的约束。笔者利用LA-ICP-MS锆石U-Pb定年,获得拉浪麦英云闪长岩加权平均年龄为421.5±5.0 Ma(n=21,MSWD=0.34),厘定其形成时代为顶志留世。岩石地球化学研究表明,英云闪长岩为弱过铝质低钾拉斑—钙碱性系列岩石,成因为Ⅰ型花岗岩类型。岩石明显富集大离子亲石元素(Rb、Sr、K)、活泼的不相容元素(如U、Th)和Pb,相对亏损高场强元素(如Nb、Ta、P、Ti)。Rb/Sr、Nb/Ta特征反映出岩石具壳幔混合岩浆的特点。结合东昆仑地区已有年代学资料和区域地质构造演化特征,认为拉浪麦英云闪长岩形成于原特提斯构造旋回的后碰撞阶段。综合分析认为加里东晚期也是东昆仑地区一个重要的多金属成矿期,在今后的找矿工作中应引起足够的重视。     

西藏班公湖-怒江成矿带主碰撞期成矿作用:荣嘎钼矿岩石地球化学及同位素年龄的证据

荣嘎钼矿是新近在班公湖-怒江成矿带发现并证实的首例斑岩型钼矿床。为了查明班公湖-怒江成矿带主碰撞期的成岩成矿作用,探讨荣嘎钼矿的富Mo机制,对该矿床进行岩石地球化学以及同位素年代学的研究。本次研究获得荣嘎含矿二长花岗斑岩的锆石SHRIMP206Pb/238U加权平均年龄为99. 8±1. 9Ma (MSWD=2. 0),属晚白垩世早期。7件辉钼矿样品的模式年龄较为一致,介于98. 0±1. 6Ma～101. 8±1. 7Ma之间,加权平均年龄为99. 7±1. 1Ma。荣嘎含矿二长花岗斑岩样品主量元素表现为富硅(SiO_2=72. 83%～75. 49%)、富碱(Na_2O+K_2O=8. 12%～8. 55%)、过铝质(A/CNK=1. 08～1. 13)的高钾钙碱性系列岩石。样品明显富集Rb、K、Th、U等元素,中等亏损Nb、Ta,强烈亏损Ba、Sr、P、Ti等元素,具中等程度的负Eu异常(δEu=0. 53～0. 63),稀土元素配分曲线呈轻稀土相对较陡,重稀土较为平坦的右倾型。荣嘎含矿二长花岗斑岩样品的(~(87)Sr/~(86)Sr)i值为0. 7042～0. 7055,ε_(Nd)(t)值介于0. 03～0. 68之间,锆石ε_(Hf)(t)值介于+12. 4～+17. 9之间,两阶段模式年龄t_(DM2)较为年轻(31～314Ma)。基于以上同位素以及岩石地球化学特征研究,荣嘎含矿二长花岗斑岩很可能是幔源岩浆诱发了富Mo洋底沉积物的熔融并与壳源熔体混合形成母岩浆,再经历结晶分异作用而形成。结合本文以及区域上的研究资料,荣嘎钼矿可能形成于拉萨地块与南羌塘地块碰撞的大地构造背景,富Mo大洋沉积物熔体的加入及随后的结晶分异是荣嘎含矿岩浆中金属Mo富集的主要原因。班公湖-怒江成矿带南缘侏罗-白垩纪海相地层分布的地区,可能为勘查钼矿床的有利靶区,应重点评价晚白垩世早期高分异中酸性岩体的含矿性。     

江绍断裂带晚中生代镁铁质火山岩成因及其深部过程意义

本文对沿江绍断裂带的江山和浦江两地区中生代镁铁质火山岩进行了Ar-Ar年代学、岩石学和元素-同位素地球化学研究。Ar-Ar年代学结果显示,江山玄武岩的形成年龄约为99Ma,为晚白垩世喷发产物;浦江玄武岩和玄武质安山岩的喷发时间为111～112Ma,属于早白垩世。两区中生代镁铁质火山岩都为中钾钙碱性系列,根据其地球化学指标可划分为三组:第1组为江山OIB型玄武岩,在微量元素组成上无Nb-Ta亏损和Pb负异常,高ε_(Nd)(t)和低(~(87)Sr/~(86)Sr)_i比值,总体上显示出类似洋岛玄武岩的元素-同位素组成特征;第2组为高钛磷玄武岩,Nb-Ta弱负异常和明显Pb正异常,中等ε_(Nd)(t)和(~(87)Sr/~(86)Sr)_i比值;第3组为低钛磷玄武岩,强烈Nb-Ta负异常和中等Pb正异常,低ε_(Nd)(t)(-6.0～-3.7)和高(~(87)Sr/~(86)Sr)_i为特征。三组镁铁质火山岩相互之间缺乏分异演化关系,也不是同一母岩浆经历地壳混染作用的结果。对比三组镁铁质火山岩的La/Nb、Ba/Nb、Zr/Ba比值和Sr-Nd同位素模拟结果,我们认为江绍断裂带晚中生代玄武岩为软流圈-岩石圈相互作用的产物,从早期(111～112Ma)到晚期(99Ma)镁铁质岩浆地幔源区软流圈组分的比例越来越多而岩石圈成份逐渐减少的趋势。考虑到该区晚中生代的盆岭构造格局与邻区郴州-临武断裂带同时期镁铁质岩浆的产出特点,华南地区在早白垩世晚期到晚白垩世期间发生了强烈的岩石圈伸展-减薄作用。     

东准噶尔乌尊塔格辉长岩LA-ICP-MS锆石U-Pb年龄、地质地球化学特征及构造意义

乌尊塔格辉长岩位于东准噶尔地区卡拉麦里造山带东段,卡拉麦里蛇绿混杂岩带北侧。岩性为辉长岩、石英闪长岩。岩石化学成分显示,辉长岩具高Al2O3(18.87%~14.11%)、CaO(8.64%~12.35%),低碱K2O(0.34%~0.86%)、Na2O(1.68%~3.08%)、P2O5(0.06%~0.20%)和TiO2(0.96%~1.76%)特征。岩石具弱富集大离子亲石元素Rb,K,Sr,Ba,显著亏损Nb,Ta,反映源区为受俯冲流体交代改造的岩石圈地幔。锆石LA-ICP-MS测年结果表明,辉长岩形成年龄为(337±17) Ma(MSWD=2.3, n=6)。结合地质背景认为,乌尊塔格辉长岩形成于俯冲结束之后挤压环境向拉张环境过渡的构造环境。     

新疆碱泉子一带水系沉积物测量地球化学特征及找矿远景

在1∶5万水系沉积物测量基础上,对新疆碱泉子一带成矿元素分布特征进行研究,认为研究区寻找Au,Cu,Zn,B矿潜力较大。通过元素异常特征,结合成矿地质条件、异常查证结果及已知矿点信息,认为该区成矿类型主要有3类,即构造破碎蚀变岩型金矿、火山岩型铅锌矿、火山岩型铜硼矿,划分出恰坎达坂金铜硼成矿远景区,阿克克尔金铜铅锌多金属成矿远景区。     

新疆卡拉麦里苏吉泉东斜长花岗岩岩石地球化学特征、成因及构造环境

新疆卡拉麦里蛇绿混杂岩带中广泛存在斜长花岗岩,对其形成时代及成因尚存较大争议。岩石地球化学显示该花岗岩具高硅、富铝和钠,低镁和钾特征;轻稀土富集,具Eu的负异常(δEu为0.55~0.78)。岩石富Ba,Th,亏损 Nb,Ti 等。对该苏吉泉东斜长花岗岩的成因研究进行探索,认为该斜长花岗岩是在低压低温条件下镁铁质洋壳发生部分熔融的产物,残留相为斜长石+角闪石±斜方辉石±钛铁矿(无石榴子石)。结合区域地质资料,苏吉泉东斜长花岗岩可能是处于早石炭世后碰撞背景下的一次构造-热事件产物,而非是蛇绿岩中的浅色岩。     

随机岩溶介质的电阻率正演模拟

受岩溶发育的影响,岩溶地区电阻率分布具有很大的不确定性。采用传统的正演方法在一定程度上不能满足岩溶探测的需要。为了更真实地反映岩溶介质的电性特点,本文提出了一种根据随机过程的谱分解理论来构造随机介质模型的方法,并以此建立了岩溶地区随机地电模型。对理论和实测地电模型的正演计算表明,随机介质能更有效模拟实际地电结构,正演结果更接近实测的异常形态。因此,岩溶地区的电阻率正演模拟计算采用随机介质模型是十分有必要的,一方面有助于对异常的认识,另一方面,随机介质模型可以更好地模拟岩溶地区的地电模型。      

Geochemistry and Hf Isotopes Characteristics and Geological Significance of Latest Early Permian Granodiorite of Langshan Area,Inner Mongolia

The Langshan area is located in the west of Inner Mongolia, southwest of the Solonker ophiolite belt, which is the ideal workplace to study tectonic-magmatic evolution and geodynamics background in Late Paleozoic because of the intense magmatism. LA-ICP-MS Zircon U-Pb ages yielded the formation time of granodiorite of Sharichulu and Moruogu granitic pluton (278.07±0.66) Ma and (278.05±0.69) Ma, respectively, forming in the latest early Permian. Geochemical characteristics showed that the Zhalashan granites were high-K calc-alkaline (SiO₂：63.76%～67.88%;K₂O：2.74%～4.02%) and weak peraluminous rocks (A/CNK：0.94～1.07) with moderate Mg# values (38.98～47.53). In addition,it exhibited LILE and LREEs enriched, HFSE and HREEs depleted with slight Eu anomaly, and all of the above-mentioned indicated the characteristics of I-type granites and arc-related. Zircon Hf isotope showed that the scatter εHf（t）values varied from -0.6～-7.5, which suggests a mixed magma source of old continental crust with a small involvement of mantle components. Together with regional geological background,the authors hold that the Latest early Permian granodiorite of Langshan area formed in an active continental margin setting under the background of southward subduction of the Paleo-Asian Ocean which were not close at that time.