Data for : The role and significance of juvenile sediments in the formation of A-type granites, West Junggar oceanic arc (NW China): zircon Hf-O isotopic perspectives

The mafic enclaves from Mesozoic intermediate-acid magmatic rocks, widely developed along Fujian coast, are considered to be the results of large-scale crust-mantle interaction and magma mixing. In this paper, petrography, mineralogy, and geochemistry of granites and mafic microgranular enclaves (MMEs) in Langqi Island are studied to provide new information for tracing crust-mantle interaction. The zircon U-Pb dating results show that the Langqi rocks were formed at ~101 Ma, which are metaluminous, enriched in silica and high-K calc-alkaline I-type granites. The enclaves have a typical magmatic structure, which is characterized by magma mixing between high-temperature basic magma and low-temperature acidic magma through injecting. The enclaves and host granites show a tendency to mixed major and trace elements, displaying a clear-cut contact relationship, which is indicative of coeval magmatism. The genesis of Langqi rocks is related to the extensional setting caused by the subduction of Paleo-Pacific Plate, and they are the results of mixing of subduction-related metasomatized mantle-derived mafic and induced crustal-melted granitic magma originating from partial melting of the crustal material.

In the center of Iran, Central-East Iranian Microcontinent (CEIM) was a part of Gondwana supercontinent in pre-Palaeozoic during Pan-African Orogeny. It is a zone of several tectonomagmatic and metamorphic episodes from Neoproterozoic to earliest Palaeozoic. In the north of CEIM, Airekan granite is a relic of Paleozoic magmatism in northern Gondwanaland. It is potentially a significance pluton that preserved the magmatic/metamorphic evolution of the active continental margin of the vanished Ocean of Proto-Tethys. This pluton is characterized by SiO 2 > 70 wt%, A/CNK>1, Rb >~160 ppm, Y 5. The δ 18 O value of quartz (average ~11.86 ‰; n=8), calculated δ 18 O value of whole rock (average ~10.75 ‰), absence of hornblende, presence of biotite, muscovite and inherited zircon, higher content of orthoclase, and microgranular granitic enclaves are all consistent with it being a continental collision-related peraluminous S-type granite. Th+U versus 206 Pb/ 238 U ratios of zircons correlate with decreasing crystallization temperatures related to the Cambrian-Ordovician magmatic events preserved in the inherited and magmatic zircons, toward their Devonian metamorphic overgrowth occurred via Caledonian Orogeny. It is probably formed by mica-dehydration melting at ~ 690-820 °C/ 10-15 kbar, and it is geochronologically and geochemically comparable with other Gondwanan collision-related granitic plutons (along north of Africa, Turkey, Iran to Himalaya).

盖吉夏铜多金属矿床位于香格里拉地区格咱斑岩带红山-普朗铜多金属成矿亚带北段，铜多金属矿化与石英二长闪长玢岩紧密相关。石英二长闪长玢岩LA-ICP-MS锆石U-Pb年龄为216.6±2.3Ma（MSWD=1.9），与区域上普朗、松诺、地苏嘎、雪鸡坪、春都等含矿斑岩体年龄一致。锆石eHf（t）值分布范围为-1.55~3.27，平均为1.57，分布较集中；单阶段Hf模式年龄（tDM1）变化范围为733~918Ma，平均为795Ma，二阶段Hf模式年龄（tDM2）为923~1178Ma，平均为1015Ma，揭示其源区可能为亏损地幔或新生壳源物质。岩石SiO2为61.08%~69.55%，Na2O+K2O为7.22%~7.82%，Na2O/K2O为0.87~1.87，具高钾钙碱性准铝质-弱过铝质（A/CNK=0.75~1.34）特征，岩石富集轻稀土元素[（La/Yb）N=13.35~24.85]和大离子亲石元素（Rb、Ba、Pb等），相对亏损重稀土元素和高场强元素（Ti、Nb、P等），且具有高Sr（>400×10-6），低Y（< 18×10-6）、Yb（< 1.9×10-6）及高Sr/Y、La/Nb等特点，显示出正常弧岩浆系列钙碱性岩石特征，说明含矿石英二长闪长玢岩与岛弧环境密切相关。这种特征的花岗岩可能为盖吉夏的Cu成矿作用提供了重要的物质来源。

吕梁地区在华北克拉通前寒武纪研究中具有重要位置，出露大量的古元古代变质表壳岩和花岗质岩石，对研究华北克拉通古元古代地质演化历史具有重要意义。本次研究选择吕梁地区白家滩花岗片麻岩进行锆石和独居石U-Pb年代学以及锆石Hf同位素研究，2个花岗片麻岩的岩浆锆石U-Pb年龄分别为2182±16Ma和2185±24Ma，代表了其侵位时代。独居石U-Pb年龄分别为1898±7Ma和1899±14Ma，明显比锆石增生边的谐和207Pb/206Pb年龄（2180~2032Ma）年轻，说明独居石对后期变质作用的响应程度比锆石强，其U-Pb年龄更能反映白家滩花岗片麻岩经历了~1900Ma的退变质作用，与华北克拉通中部造山带的变质作用时间一致。花岗片麻岩的锆石Hf同位素亏损地幔模式年龄（tDM）为2473~2598Ma，两阶段亏损地幔模式年龄（tDMC）分别为2646~2839Ma，eHf（t）值分布于-1.3~+1.8之间，未显示同期幔源物质的加入，而是新太古代地壳物质部分熔融的产物，结合已有的古元古代中期（2.2~2.1Ga）的岩浆岩锆石Hf同位素数据，华北克拉通新太古代地壳在2.2~2.1Ga期间发生了广泛的重熔作用，这期岩浆活动在华北克拉通吕梁、中条、五台以及胶-辽-吉等地区广泛发育，可能形成于陆内裂谷环境。

通过对中亚造山带东段南缘发育的解放营子韧性剪切带的构造学研究，揭示出该地区岩石圈减薄后发生了一期强烈的伸展变形事件。野外观测和岩相学分析显示该韧性剪切带呈北东-南西走向，变形带内发育有大量的A型褶皱，矿物和砾石拉伸线理以及同构造花岗质岩墙。S-C组构、σ型角闪石残斑、压力影构造、斜长石书斜构造以及云母鱼等显微构造，指示该韧性剪切带为右旋剪切。多晶石英的波状消光、晶粒边界迁移重结晶、多晶石英条带等显微变形组构表现出中温（300~500℃）的变形条件。动态重结晶颗粒的粒径统计分析和岩石有限应变分析显示该韧性剪切带形成于一个地壳中等层次的伸展变形环境。剪切带内同构造白云母40Ar/39Ar阶段加热同位素年代学分析以及同构造花岗岩的锆石U-Pb同位素年代学测试显示，该韧性剪切带的伸展变形时代为晚三叠世（219~227Ma）。该期伸展变形事件在华北板块北缘和中亚造山带形成了低硅型和高硅型两种花岗质岩浆的侵入。其中中亚造山带内发育的低硅型岩石的岩浆源区为亏损型地幔岩石圈，并进一步演化出高硅型岩石；而华北板块北缘发育的低硅型岩浆起源于富集型岩石圈地幔，同时混入了亏损的软流圈地幔组分。

龙陵-瑞丽早白垩世流纹岩带断续延伸约100km，其形成的构造背景是揭示怒江洋演化及腾冲与保山地块关系的重要证据。笔者在野外调研的基础上，对本区流纹岩开展了岩石学、地球化学、锆石U-Pb定年和Hf同位素等方面的研究。结果表明流纹岩具有高硅贫钠、高钾钙碱性和高分异特征；锆石LA-ICP-MS U-Pb年龄分布在129~123Ma区间，代表流纹岩的喷发时代。流纹岩锆石eHf(t)值分布于-12.3~-6.3之间，对应的地壳模式年龄为1591~1826Ma；其微量元素蛛网图、稀土元素配分模式图和Nb/Ta比值、Th/U比值等与大陆中上地壳相似；岩石中无斜长石斑晶、Sr含量极低以及在Al2O3/(MgO+FeOT)]-[CaO/(MgO+FeOT)]和Rb/Ba-Rb/Sr图解上，样品都分布在变泥质岩部分熔融区；表明流纹岩岩浆来源于古老地壳物质部分熔融的产物，其低钠是源区中斜长石含量低或斜长石未参与部分熔融造成的。流纹岩轻稀土元素和大离子亲石元素(Th、U和K)富集，Nb、Ta、Ti、P和Sr强烈负异常；在构造环境判别图解上样品全部落在活动大陆边缘区；在空间上，这些流纹岩呈带状分布，构成俯冲增生杂岩带的一部分，并与同时代侵入岩紧密相邻，成因上类似于区域内同期S型花岗岩。这些特征表明研究区内流纹岩与高黎贡构造带内同时代花岗岩类相似，形成于俯冲的大陆边缘弧环境，即幔源岩浆底侵至下地壳，这一过程提供了足够的热源使壳源物质(可能为富云母、贫斜长石的变沉积岩类)发生部分熔融生成酸性岩浆，它们在经历了高度的结晶分异后喷出地表形成低钠流纹岩，并在弧前或弧间盆地内被浅海相碎屑沉积物覆盖。

东南沿海地区发育具有'等同位素组成'特征的晚中生代双峰式火成岩，其成因备受争论。本文选择了福建省平潭岛双峰式杂岩体中的花岗闪长岩、英云闪长岩与花岗岩开展了高精度锆石U-Pb定年、原位O-Hf同位素和黑云母矿物化学方面的研究，以探讨这些花岗质岩石的成因。分析结果显示，花岗闪长岩、英云闪长岩与花岗岩的锆石U-Pb年龄分别为116.8±1.0Ma、116.3±1.0Ma与117.4±1.0Ma，表明它们均侵位于早白垩世。这三类岩石也有较为相似的锆石O-Hf同位素组成。其中，锆石的δ18O分别变化为4.6%~5.3‰、4.8%~5.3‰及5.0%~5.6‰，与地幔的O同位素值基本一致。锆石的eHf（t）变化范围分别为+2.0~+7.1、+2.8~+6.5及+1.8~+5.6，相应的两阶段模式年龄tDM2分别为741~1046Ma、754~995Ma及815~1058Ma。锆石O-Hf同位素数据反映其熔融源区主要为相对较年轻的地壳物质，来自华夏地块古老基底的贡献较少。另外，黑云母矿物化学暗示这三类岩石具有Ⅰ型花岗岩的特征。但是，黑云母种属在不同岩性间存在差异，其中花岗闪长岩与英云闪长岩中的黑云母为镁质类型，花岗岩中的黑云母为铁质类型。从花岗闪长岩、英云闪长岩到花岗岩，黑云母的结晶温度和岩浆体系的氧逸度均逐渐降低，这与钙碱性岩浆分异演化的趋势基本一致。结合前人研究资料及区域地质演化历程，我们认为平潭岛杂岩体中的花岗质岩石形成于古太平洋俯冲背景，其熔融源区主要为新增生地壳物质。花岗质岩石内部岩性的差别主要是母岩浆经历不同程度分异演化的结果。我们的研究结果暗示壳幔岩浆混合在东南沿海早白垩世长英质岩石形成过程中所具有的作用可能较为有限。

北山造山带位于中亚造山带南缘，连接了东天山和索伦缝合带，对认识中亚造山带的构造演化具有十分重要的意义。本文选择北山造山带西段马庄山地区的花岗闪长岩、钾长花岗岩和闪长岩进行岩石学、地球化学、Nd-Hf同位素及LA-ICP-MS锆石U-Pb年代学的研究。结果表明，英云闪长岩、花岗闪长岩、钾长花岗岩具有高硅（64.54%~78.48%）、高钾（1.53%~4.95%）、低磷（0.01%~0.21%）的特征，显示出高钾钙碱性-钙碱性岩石的特点。岩石整体相对富集Rb、K、Th和U大离子亲石元素，亏损P、Ta、Nb和Ta高场强元素，与弧火成岩的微量元素特征一致。花岗闪长岩和英云闪长岩的SiO2与P2O5呈负相关关系，Rb与Y、Th均呈现正相关关系，说明其属于I型花岗岩。钾长花岗岩和花岗闪长岩的LA-ICP-MS锆石U-Pb年龄分别为317.7±1.0Ma和320.2±0.8Ma，表明岩浆侵位时代为晚石炭世。它们的初始eNd（t）值介于-5.31~-4.24之间，锆石eHf（t）介于-3.6~-0.2之间，Hf的tDM2为1318~1537Ma，暗示岩石来自于下地壳物质的部分熔融，原始岩浆在上侵过程中有部分地幔物质的加入。综合地质、地球化学和年代学特征，认为北山西段，在晚石炭世，马庄山地区处于与俯冲相关的弧环境。

Within the sedimentary-volcanoclastic succession known as Bithia Unit (SW Sardinia) a decameter-wide albitite lens has been found and is here characterized from a geological, petrographic and geochemical point of view. Albitite is a yellowish, fine-grained body elongated and foliated according to the main schistosity of the hosting rocks (N20-25° - 40° NW). The rock is made up by plagioclase porphyroclasts in a fine-grained matrix of plagioclase (up to 80%) and quartz; both porphyroclasts and small grains in matrix are almost pure albite (Ab 98 ). Accessory phases are ilmenite, rutile, monazite, apatite and zircon. Bulk-rock chemical analyses show that the relative proportions of SiO 2 , Al 2 O 3 and Na 2 O strongly resemble those of pure albite. Major, trace and rare earth element (REE) contents strongly suggest geochemical affinity with the nearby Ordovician metavolcanics and orthogneiss from Capo Malfatano area described in literature. Furthermore, REE, Rb/Sr, Y/Ho and Zr/Hf ratios seem to exclude the involvement of a significant Na-rich fluid metasomatism.

出露于辽东地区盖州-庄河一带的榆树砬子岩组，作为该区不整合覆盖于古元古代辽河群之上最早的稳定沉积盖层，其沉积时代、物源区及沉积环境的研究对了解华北克拉通的构造演化具有重要科学意义。目前该地区依然缺乏关于榆树砬子岩组形成时代和区域地层对比的数据。本文在详细野外地质调查基础上，通过碎屑锆石U-Pb定年、Hf同位素示踪等手段，对该岩组的沉积环境及时限、物源区及与燕辽裂谷带内中元古代地层的时空关系等问题进行了讨论。通过对榆树砬子岩组与燕辽裂谷带内常州沟组对比，发现尽管锆石U-Pb定年结果表明榆树砬子岩组沉积时代晚于1803Ma，但二者在锆石年龄谱、物源成分、岩石组合及沉积旋回序列等方面具有一定相似性。龙岗地块及辽南陆块太古宙基底、古元古代辽吉花岗岩以及古元古代同碰撞或碰撞后岩浆岩可能为榆树砬子岩组提供了物源。同时，锆石Hf同位素特征表明华北克拉通东北缘记录了自~4.1Ga以来的一系列地壳生长事件，其峰期主要发生于31~27亿年，与华北克拉通其他地区的数据一致，为华北克拉通太古代的陆壳演化提供了新的佐证。

SHRIMP U-Pb Zircon Dating and Volcanic Process of the Volcanic Rocks around the Jayang Caldera, Northern Yeongcheon, Korea

黑龙江西部龙江地区位于中亚造山带东段，黑河-贺根山缝合带与西拉木伦缝合带之间，地层记录了两大古板块之间古亚洲洋闭合过程的信息。本文对龙江地区乐平统林西组和下-中三叠统老龙头组的砂岩样品进行碎屑重矿物和碎屑锆石U-Pb同位素年代学研究。碎屑重矿物组合以锆石+磷灰石+金红石+角闪石+绿帘石+重晶石的组合为特征，表明物源主要来自于中酸性岩浆岩，并有少量变质岩及沉积岩组分。林西组样品最年轻的锆石年龄为278±3Ma，老龙头组样品最年轻的锆石年龄为247±3Ma、243±4Ma及237±3Ma，结合前人的研究，限定了林西组沉积于乐平世，老龙头组沉积于早三叠世-中三叠世。碎屑锆石年龄谱明显分为五组：237~258Ma、270~329Ma、357~558Ma、680~1633Ma及1893~1966Ma。其中237~258Ma的碎屑锆石主要来自与古亚洲洋洋壳消亡前的俯冲增生过程相关的火山活动，270~329Ma的碎屑锆石主要来自大石寨组火山岩及其同期侵入岩，357~558Ma的碎屑锆石来自早古生代-晚古生代早期岩浆弧，680~1633Ma的碎屑锆石可能来自兴安及额尔古纳地块的变质基底，而较古老的~1800Ma的锆石年龄暗示了华北克拉通基底的物源信息。通过研究发现林西组及老龙头组样品前30%年轻的碎屑锆石年龄与地层沉积年龄之差都小于100Ma，结合对砂岩碎屑组成、重矿物组合及盆地与火山弧位置关系的研究，认为研究区乐平世-中三叠世沉积盆地具有汇聚背景，为弧前盆地。

班公湖-怒江结合带是藏北一条重要的特提斯主洋残余，在洋盆消减过程中发生了强烈岩浆作用，在其南缘形成了沿狮泉河-永珠缝合带两侧广泛分布的白垩纪大规模岩浆岩和火山岩，为了解班公湖-怒江特提斯洋俯冲和闭合过程提供了良好窗口。本文对出露于革吉地区帕阿花岗闪长岩岩基进行了详细的野外地质填图、全岩元素地球化学、锆石U-Pb年代学和Hf同位素组成研究。研究结果表明，花岗闪长岩和其闪长质包体的锆石206Pb/238U年龄分别为155.7±1.0Ma（谐和年龄，MSWD=0.26，N=18）和156.5±2.3Ma（年龄加权平均值，MSWD=2.6，N=16），岩浆侵位于晚侏罗世，不是以往所认为的白垩纪岩浆产物。花岗闪长岩主要属于高钾钙碱性系列，高铝饱和指数（A/CNK=0.91~1.04），显示了过铝质特征。花岗闪长岩富集轻稀土元素（LREE/HREE）=5.51~8.54，（La/Yb）N=6.27~20.2）和大离子亲石元素，亏损Nb、Ta、Sr、Ti等高场强元素，具有中等至弱Eu负异常（δEu=0.61~0.85）。锆石Hf同位素分布集中，花岗闪长岩和闪长质包体的锆石eHf（t）均为负值，分别为（-9.4~-6.3）和（-10.9~-8.6），Hf地壳模式年龄分别为（1.32~1.48Ga）和（1.44~1.56Ga），指示了帕阿花岗闪长岩岩基主体系古老地壳基底部分熔融产物，存在少量地幔物质的加入，形成于弧环境，可能属于班公湖-怒江洋盆南向俯冲到拉萨地块古老结晶基底重熔形成的一套具弧属性的未分异Ⅰ型花岗岩。

佛子岭群为北淮阳变质单元的重要组成部分，其位于大别碰撞造山带最北缘，处于扬子和华北板块的结合部位，构造位置极为关键。长期以来，佛子岭群一直被视为扬子板块北缘沉积产物，并以加积楔形式产出。然而，随着年代学和构造地质学的研究的深入，对其构造归属和形成动力学机制产生了差异性认识，究其原因主要是对佛子岭群变质属性缺乏准确的判定所致。为此，本研究对佛子岭群展开了详细的野外观测和区域变质岩石学研究，以及岩相学、热力学和年代学分析。研究表明佛子岭群构成较为复杂，主体由细粒的云母石英片岩、石英云母片岩和石榴云母片岩构成。主期变质PT条件评价显示，其温压范围为610~620℃和0.9~1.0GPa，普遍经历了中-高压角闪岩相变质，并非为'浅'变质产物，并卷入了较深层次的俯冲-折返过程。锆石U-Pb年代学研究表明，佛子岭群年龄跨度较大，范围为3101±48Ma~413±9Ma，可分为5组年龄，年龄峰值分别为2537Ma、1567Ma、940Ma、749Ma和440Ma，显示了华北和扬子板块混合物源特征，最年轻的年龄峰值暗示其形成于早古生代晚期。

LIU Shen, FENG Caixia, M. Santosh, FENG Guangying, XU Mengjing, Ian M. COULSON, GUO Xiaolei, GUO Zhuang and FAN Yan

New Zircon U‐Pb Age of Devonian Granites in the Niukutou Lead‐Zinc Deposit, Qinghai Province and its Significance for Prospecting Blind Orebodies

O Granito Marajoara (GrMj) é um stock intrusivo em granitoides mesoarqueanos do Domínio Rio Maria (DRM) da Província Carajás (PC). É formado por rochas leucocráticas: biotita monzogranito equigranular (BMzE) e heterogranular (BMzH). Textura rapakivi e enclaves de granito porfirítico (EGp) e microgranulares (EMg) são restritos à fácies BMzH. Os valores de susceptibilidade magnética (SM) e a presença de magnetita aproximam a fácies BMzH dos granitos da série magnetita, enquanto a variedade BMzE mostra afinidade com aqueles da série ilmenita por apresentar baixos valores de SM e opacos. São granitos peraluminosos similares aos ferroan. Mostram, ainda, afinidades com os granitos tipo-A oxidados (BMzH e EGp) e reduzidos (BMzE), como aquelas dos plútons das suítes Jamon e Velho Guilherme, respectivamente. Os EMg mostram afinidade com os granitos magnesianos e da série calcioalcalina, e os EGp são originados a partir da interação entre os líquidos EMg e BMzH. Os gaps composicionais entre as diversas variedades sugerem que seus magmas não são cogenéticos. Os EMg são representantes de um magmatismo básico oriundo do manto litosférico enriquecido e que teriam sido injetados na câmara magmática por underplating. As análises U-Pb em zircão (SHRIMP) fornecem idade de cristalização (1885 ± 6 Ma). O GrMj foi colocado em epizona em um ambiente de tectônica extensional. A zonalidade concêntrica do GrMj e a influência reduzida dos esforços regionais indicam que o transporte do magma se deu por diques. Dessa forma, a edificação do GrMj é resultante de ascensão vertical de magmas por meio de fraturas e acomodação ao longo dos planos da foliação regional E-W, seguida de uma mudança do fluxo vertical por um espalhamento lateral do magma.

The suprasubduction Bulfat Igneous Complex in NE Iraq is one of the discontinuous Neotethys oceanic lithosphere fragments preserved in the Zagros region. Reported here are new geochronological, petrological and geochemical data from this assemblage. At the study locality, Eocene arc-related magmatic rocks are intruded by extensive dykes of Shaki-Rash gabbro. The key reason to focus on these rocks is the remarkable lack of hydrothermal alteration, including preservation of very fresh olivine. Besides olivine the gabbros contain plagioclase and clinopyroxene, with lesser orthopyroxene, biotite, brown hornblende and alkali feldspar. From mineral chemistry, the crystallization conditions of the olivine gabbros are ~ 950-1050°C and ~ 5-3 Kbar. Shaki-Rash gabbro shows alkaline affinity. SHRIMP U-Pb dating of magmatic zircon from the gabbro yielded a mean 206Pb/238U age of 39.0±0.5 Ma (Eocene). Most of the Shaki-Rash gabbro samples have Ti/V of 100 to 50 and fall in the back-arc basin (BAB) and alkaline fields, while on a Nb/Yb-Th/Yb diagram, most samples fall within the MORB-OIB array and plot in proximity to E-MORB. Primitive-mantle-normalized trace-element patterns show enrichment in the large ion lithophile elements without depletion of the high-field-strength elements. These geochemical signatures and their field setting advocate for the formation of these dikes in an extensional tectonic environment, such as an intra-arc rift.

New geological mapping and U-Pb dating of detrital zircons and volcanic rocks permit stratigraphic revision and interpretation of pre- Late Permian deformation in the Sierra Plomosa area, located 95 km east of the city of Chihuahua and at comparable distance to the southwest of the border town Ojinaga. The Plomosas area forms the central part of a north-northwest trending uplifted block ranging from Placer de Guadalupe at the north, to the cerro Carrizalillo at the south. Along this elevated block, Precambrian crystalline basement rocks are exposed, as well as lower Paleozoic rocks belonging to the southern platform of Laurentia and upper Paleozoic to lower Mesozoic clastic and volcaniclastic rocks. The main structures in the area are north-northwest trending folds and thrust faults showing a west-southwest vergence or tectonic transport, being the age of this deformation, necessarily pre-Late Permian, given the occurrence of an erosional and angular unconformity between strongly deformed pre-Permian strata and less deformed Permian-Triassic clastic and volcaniclastic deposits which contain Archean (3,022–2,513Ma), Paleoproterozoic (2,461–1,601Ma), Mesoproterozoic (1,592–1,013Ma), Neoproterozoic (994–546Ma) and Paleozoic (533–252Ma) detrital zircon populations, with maximum depositional ages calculated between 252 and 244 Ma and isolated concordant zircons as young as 227 to 223 Ma. In the middle part of the permo-triasic succession, a Permian rhyolitic flow or ignimbrite was previously reported on the basis of a lead alpha age, however, during the present research this rock yielded an Early Jurassic age of 183.3+2.2/–2.7 Ma. On the basis of petrographic features and field relations, the rhyolite is interpreted as an ash flow tuff or ignimbrite, similar to other rhyolitic flows observed in the area. Such flows are concordant to bedding and several clasts of this rock are present in strata overlying the rhyolite. All Paleozoic rocks, including the top of the Permian and Lower Triassic, underlie clastic and volcaniclastic Jurassic rocks of shallow marine origin. In the area was also documented an erosional, locally angular unconformity which separates a Lower Jurassic unit from a Middle (?) to Upper Jurassic unit with previously reported Late Jurassic fauna in the upper part of the succession. Limestone layers in the upper part of the Jurassic succession, are recrystallized as marbles, especially in the area of the Plomosas mine, where both the metamorphism of these limestones and the presence of stratabound ore bodies of lead and zinc seem to be related to thrusting with a tectonic transport to the southwest and northwest, that placed very fractured Permian sandstones and siltstones over the Upper Jurassic rocks. The faulting is possibly associated with transpression along the so-called Plomosas lineament and transcurrent faulting, described as a left lateral shear of the basement whose regional trace lies along the eastern front of the sierra. Finally south of the Sierra Plomosa, in the northeast trending Sierra de Santo Domingo Carboniferous and Permian limestone are thrust over Jurassic sandstones and shale. This thrust fault also cuts a rhyolitic dyke of probable Oligocene age and can be considered as a very late laramide shortening contractive structure, possibly related to transpressive deformation along the Plomosas lineament and its interaction with the uplifted Cerro Carrizalillo block 15 km to the South of Plomosas.