Early Miocene post-collisional magmatism in NW Turkey: geochemical and geochronological constraints

The Alaçam region of NW Turkey lies within the Alpine collision zone between the Sakarya continent and the Menderes platform. Four different tectonic zones of these two continents form imbricated nappe packages (including the Afyon zone), intruded by the Alaçam granite. Newly determined U-Pb zircon ages of this granite are 20.0 ± 1.4 and 20.3 ± 3.3 Ma, indicating early Miocene emplacement. Rb-Sr biotite ages of the granite are 20.01 ± 0.20 and 20.17 ± 0.20 Ma, suggesting fast cooling at a shallow crustal level. Geochemical characteristics show that the Alaçam granite is similar to numerous EW-trending plutons in NW Anatolia.

Gneissic granites of the Afyon tectonic zone were intruded by the Miocene Alaçam granite and have been interpreted in earlier studies as sheared parts of the Alaçam granite, which formed along a crustal-scale detachment zone under an extensional regime. We determined a U-Pb zircon age of 314.9 ± 2.7 Ma for a gneissic granite sample of the Afyon zone, demonstrating that these rocks are unrelated to the Miocene Alaçam granite. The early Miocene granitic plutons bear post-collisional geochemical features and are interpreted as products of Alpine-type magmatism along the Izmir–Ankara suture zone in NW Turkey, and seem to have no genetic relation to the detachment zone.     

Zircon U–Pb and pyrite Re–Os age constraints on pyrite mineralization in the Yinjiagou deposit,China

We report new zircon U–Pb and pyrite Re–Os geochronological studies of the Yinjiagou poly-metallic deposit, sited along the southern margin of the North China Craton (SMNCC). In this deposit, pyrite, the most important economic mineral, is intergrown/associated with Mo, Cu, Au, Pb, Zn, and Ag. Prior to our new work, the age of chalcopyrite–pyrite mineralization was known only from its spatial relationship with molybdenite mineralization and with intrusions of known ages. The U–Pb and Re–Os isotope systems provide an excellent means of dating the mineralization itself and additionally place constraints on the ore genesis and metal source. Zircons separated from the quartz–chalcopyrite–pyrite veins include both detrital and magmatic groups. The magmatic zircons confine the maximum age of chalcopyrite–pyrite mineralization to 142.0 ± 1.5 Ma. The Re–Os results yield an age of 141.1 ± 1.1 Ma, which represents the age of the chalcopyrite–pyrite mineralization quite well. The common Os contents are notably low (0.5–20.1 ppt) in all samples. In contrast, the Re contents vary considerably (3.0–199.2 ppb), most likely depending on intensive boiling, which resulted in an increase of Re within the pyrite. This study demonstrates that the main chalcopyrite–pyrite mineralization occurred late in the magmatic history and was linked to a deeper intrusion involving dominant mantle-derived materials. This mineralization event might be related to the Early Cretaceous lithospheric destruction and thinning of the SMNCC.     

Jurassic–Cretaceous tectonic evolution of Southeast China: geochronological and geochemical constraints of Yanshanian granitoids

We report results of laser ablation inductively coupled plasma-mass spectrometry-based dating, as well as the analysis of bulk-rock major and trace elements, and Sr–Nd isotopes to address the genesis and tectonic settings of the Yanshanian granitoids in neighbouring sections of Zhejiang, Jiangxi, and Anhui provinces (the WZG region) within the Yangtze block. Geochronological results indicate that intense magmatic activity took place during Jurassic to Cretaceous time in the WZG region. Three episodes can be clearly distinguished by their bulk-rock geochemistry. (1) Early–Middle Jurassic granitoids (180–170 Ma) have high Sr and low Yb content, high ?_Nd(t) and low initial ⁸⁷Sr/⁸⁶Sr ratios, and weakly negative Eu anomalies. These granitoids are strongly enriched with LREE, Rb, K, and Th but are depleted of HREE, Nb, and Ta. (2) Late Jurassic to Early Cretaceous granitoids (165–140 Ma) have relatively low Sr and low Yb contents, as well as low ?_Nd(t) and high initial ⁸⁷Sr/⁸⁶Sr ratios, with characteristics similar to those of the Early–Middle Jurassic granitoids in terms of the rare earth element and trace element patterns. (3) Early Cretaceous granitoids (140–120 Ma) have extremely low Sr and high Yb concentrations, as well as high SiO₂ but low MgO, CaO, and Al₂O₃ content, with strong negative anomalies in Eu, Ba, Sr, P, and Ti. These characteristics indicate that the WZG Jurassic granitoids were related to northwestward subduction of the Izanagi plate, whereas the Early Cretaceous granitoids formed in a within-plate extensional setting. The time of transition between the two tectonic environments can be constrained to ～140 Ma. This tectonic transition may be attributed to progressive slab roll-back of the Izanagi plate. The presence of two A-type granite belts in the WZG region probably reflects lithospheric thinning. The NE trend of the A-type granite belts indicates that this extension in Southeast China was controlled by underflow of the Izanagi plate.     

Prospects for gold and other economically valuable minerals in volcanogenic formations of northern Sea of Okhotsk coastal region

We review data for the Tuva–Mongolia Massif and show that this massif was not derived from the Siberian Craton.     

东天山卡瓦布拉克地区片麻状花岗岩形成时代及地质意义

东天山卡瓦布拉克地区发育大量片麻状花岗岩,据野外特征分为含斑片麻状花岗岩和不含斑片麻状花岗岩,片麻理属后期韧性剪切叠加改造结果.本文获得含斑片麻状花岗岩LA-ICP-MS锆石U-Pb年龄为(942.1±7.2)Ma和(941.9±4.9)Ma;不含斑晶的片麻状花岗岩年龄为(424.2±3.4)Ma,这些年龄数据表明片麻状花岗岩分别形成于新元古代和中志留世.野外调查表明,片麻状花岗岩侵入于长城系星星峡群和卡瓦布拉克群中,被块状石炭纪花岗岩侵入.研究区西侧上覆泥盆纪地层未发生明显韧性变形.据此限定区内韧性剪切变形发生于晚志留世(约424 Ma)之后至泥盆纪之前.     

中祁连西段巴嘎德尔基岩体LA-ICP-MS锆石U-Pb年龄及地质意义

巴嘎德尔基岩体位于中祁连山西段的野马南山地区,主要由钾长花岗岩和二长花岗岩组成.LA-ICP-MS锆石U-Pb测年表明,巴嘎德尔基岩体侵位年龄为462.9±1.4Ma.该岩体地球化学方面表现出强过铝、高K、K/Na,低Mg、Fe、Ca的主量元素特征,并富集K、Rb、Ba、Th,亏损Zr、Hf、Y、Yb等微量元素,轻稀土富集,弱的Eu负异常.在微量元素判别图解上,所有岩石投影点均落入同碰撞区域.结合区域地质背景,认为巴嘎德尔基岩体是加里东造山作用晚期陆陆碰撞的产物.     

湘南宜章地区辉绿岩、花岗斑岩、安山岩的形成时代和成因——锆石U-Pb年龄和Hf同位素组成

湘南宜章县长城岭地区广泛出露以辉绿岩、花岗斑岩为主的燕山早期岩体,而宜章平和地区主要分布安山岩体,其年龄和成因有待进一步研究。分别用SHRIMP和LA-ICP-MS法测试了它们的年龄和Hf同位素组成,长城岭辉绿岩的SHRIMP锆石U-Pb年龄为153.7Ma±3.1Ma、227.0Ma±4.2Ma;长城岭花岗斑岩的LA-ICP-MS锆石U-Pb测年结果为153Ma±14Ma和231.58Ma±0.67Ma;平和安山岩的SHRIMP锆石U-Pb测年结果为159Ma±14Ma和229.3Ma±7.6Ma,LA-ICP-MS锆石U-Pb测年结果为161Ma±1Ma,显示为燕山早期岩浆侵位和印支期基性岩浆底侵。锆石Lu-Hf同位素原位分析结果表明,εHf(t)值在正值和负值范围内变化,指示岩浆为壳幔混合来源。结合长城岭花岗岩体中继承锆石的年龄信息,认为花岗质岩石可能来源于中元古代基底的重熔。岩体岩浆很可能是由元古宙火成岩石部分熔融形成的,并伴有年轻或新生幔源物质的加入,岩浆上升侵位的过程中发生了混合作用。     

大兴安岭中段莫克河地区新生代火山活动

通过野外地质观测、岩石地球化学分析及高精度加速器质谱(AMS)14 C测年等工作,对大兴安岭中段莫克河地区新生代火山活动进行了详细研究。结果表明:莫克河地区新生代火山活动活跃,覆盖面积超过80km2,喷发方式为斯特朗博利型喷发。火山活动最早始于晚更新世,火山活动经历了4个火山喷发旋回,并在第一、二个火山旋回之间有短暂的间歇。火山岩为以低硅、高镁、高钾、高钛为主要特征的高钾钙碱性橄榄玄武岩。综合周边地区资料,研究区新生代火山岩是在拉张的构造环境下、以复活的深大断裂为通道产生的板内OIB型玄武岩,岩浆在上升过程中没有或很少发生壳源物质混染,也没有发生斜长石结晶分异,但有橄榄石、单斜辉石的分离结晶作用。     

西南天山东柯坪推覆系古地震初步研究

西南天山柯坪推覆系发育多排逆断裂-褶皱带,各排逆断裂-褶皱带的变形速率并不一致,古地震发生规律也可能具有不同特点.自汶川地震造成龙门山中央和前山两条断裂同时活动之后,对于如何判断多条逆断裂级联破裂的逆断裂古地震事件,是一个值得讨论的问题.文中通过对东柯坪塔格和萨尔干塔格两排逆断裂-褶皱带山前多期地貌面(冲洪积扇面)上发...     

广西钦甲花岗岩体单颗粒锆石LA-ICP-MS U-Pb定年及其地质意义

本文对钦甲花岗岩体进行了详细的LA-ICP-MS锆石U-Pb同位素年龄测定.这些锆石的阴极发光图像发育环带结构,Th/U值较高(大于0.1),具有岩浆成因锆石的特征.研究结果表明,钦甲花岗岩体为一复式岩体,其不同单元锆石的206Pb/238U加权平均年龄分别为412.4±1.8 Ma、434.8±1.7 Ma、442....