Thermal evolution of the Tseel terrane,SW Mongolia and its relation to granitoid intrusions in the Central Asian Orogenic Belt

The timing and thermal effects of granitoid intrusions into accreted sedimentary rocks are important for understanding the growth process of continental crust. In this study, the petrology and geochronology of pelitic gneisses in the Tseel area of the Tseel terrane, SW Mongolia, are examined to understand the relationship between igneous activity and metamorphism during crustal evolution in the Central Asian Orogenic Belt (CAOB). Four mineral zones are recognized on the basis of progressive changes in the mineral assemblages in the pelitic gneisses, namely: the garnet, staurolite, sillimanite and cordierite zones. The gneisses with high metamorphic grades (i.e. sillimanite and cordierite zones) occur in the central part of the Tseel area, where granitoids are abundant. To the north and south of these granitoids, the metamorphic grade shows a gradual decrease. The composition of garnet in the pelitic gneisses varies systematically across the mineral zones, from grossular‐rich garnet in the garnet zone to zoned garnet with grossular‐rich cores and pyrope‐rich rims in the staurolite zone, and pyrope‐rich garnet in the sillimanite and cordierite zones. Thermobarometric analyses of individual garnet crystals reveal two main stages of metamorphism: (i) a high‐P and low‐T stage (as recorded by garnet in the garnet zone and garnet cores in the staurolite zone) at 520–580 °C and 4.5–7 kbar in the kyanite stability field and (ii) a low‐P and high‐T stage (garnet rims in the staurolite zone and garnet in the sillimanite and cordierite zones) at 570–680 °C and 3.0–6.0 kbar in the sillimanite stability field. The earlier high‐P metamorphism resulted in the growth of kyanite in quartz veins within the staurolite and sillimanite zones. The U–Pb zircon ages of pelitic gneisses and granitoids reveal that (i) the protolith (igneous) age of the pelitic gneisses is c. 510 Ma; (ii) the low‐P and high‐T metamorphism occurred at 377 ± 30 Ma; and (iii) this metamorphic stage was coeval with granitoid intrusion at 385 ± 7 Ma. The age of the earlier low‐T and high‐P metamorphism is not clearly recorded in the zircon, but probably corresponds to small age peaks at 450–400 Ma. The low‐P and high‐T metamorphism continued for c. 100 Ma, which is longer than the active period of a single granitoid body. These findings indicate that an elevation of geotherm and a transition from high‐P and low‐T to low‐P and high‐T metamorphism occurred, associated with continuous emplacement of several granitoids, during the crustal evolution in the Devonian CAOB.     

新疆多拉纳萨依金矿一带花岗岩类形成时代及其对金矿成矿作用的制约

新疆阿尔泰造山带西南缘分布有多拉纳萨依、赛都、哲兰德等大中型金矿床,近年又发现沃多克等小型金矿.这些金矿主要赋存于糜棱岩化石英闪长岩中,岩石地球化学特征表明沃多克和多拉纳萨依金矿含矿石英闪长岩特征基本一致,属同一期岩浆活动的产物,而明显不同于两矿床之间的萨热乌增英云闪长岩.沃多克金矿含矿石英闪长岩和萨热乌增英云闪长岩LA-ICP-MS锆石U-Pb年龄分别为(299.4±4.1) Ma和(317.7±1.5) Ma.二者分别属于后造山和后碰撞花岗岩.沃多克含金石英闪长岩年龄限定了多拉纳萨依一带金矿形成时代不早于300 Ma,结合前人的研究成果分析,表明其主要成矿时期为290 Ma左右.萨热乌增等岩体的形成早于金矿形成时代,金的成矿作用与该期大规模花岗岩体的侵位无关.     

Role of the shale tectonics on the evolution of the Eastern Venezuelan Cenozoic thrust and fold belt

This paper presents a structural and stratigraphic analysis of the foreland-fold-belt of the Eastern Venezuelan Basin and the main conclusions about shale tectonic mechanisms in the area. The deformation of the foreland-fold-belt has been investigated analyzing the growth strata architecture preserved on the structure fold limbs. Three contractional episodes are proposed for the Eastern Venezuelan Basin: 1) Oligocene to middle Miocene, 2) late Miocene to Pliocene and 3) Pleistocene. The first episode produced contractional listric faults inside the shale and long displacement blind thrusts in the underlying Cretaceous units. The second episode produced the deformation of the Cenozoic strata into overlapping east-west-trending, convex northward anticlines that covers more than 200 kilometers in length and 40 kilometers wide, break-through normal faults product of a high sedimentary load that overcomes contraction and the formation of short-displacement blind thrusts in the underlying Cretaceous units. The last episode is related to an oblique compression and the formation of high angle extensional faults with dextral movement and NW-SE strike. The role of the shale tectonics in the evolution implies that shale deforms in two stages: 1) folding and 2) normal faulting of the crest of the anticline (Break through normal faulting). Folding controlled the sediment distribution during most of the Neogene strata, while the normal faulting of the anticlines represent basin potential for hydrocarbon. The best potential hydrocarbon plays in the basin are related to oblique-collision restricted basins and controlled by break-through normal faults and the presence of NW-SE strike faults that connect the HC source with the reservoirs. Results from this research imply that the role of sedimentation is fundamental for the overburden sand distribution and tectonic constrain of the folds.     

南岭地区中生代两种成矿花岗质岩的岩石学和矿物学特征探讨

南岭地区是我国重要的有色金属矿产资源集中区,中生代多金属成矿与本区广泛分布的花岗质岩石具有密切的成因关系.本文在总结南岭地区与花岗质岩石有关的钨锡钼铋和铜(钼)铅锌金银两类多金属矿床时空分布规律、岩体特征的基础上,探讨并对比了这两种成矿花岗质岩的岩石学和矿物学特征,指出:①与钨锡钼铋成矿有关的岩浆岩以壳源物质重熔形成的黑云母花岗岩为主,但不少岩体也带有幔源物质信息.造岩矿物组合为石英+钾长石+斜长石+黑云母+白云母±角闪石,石英、长石、黑云母包含结构十分发育.斜长石以钠长石为主,An平均值为8;黑云母为富铁黑云母—铁叶云母—铝铁叶云母,MF值为0.001～0.5311;个别岩体中含角闪石,Mg# =0.21～0.53.②与铜(钼)铅锌金银成矿有关的岩浆岩以壳幔混源的花岗闪长岩为主.造岩矿物组合为石英+斜长石+钾长石+黑云母+角闪石±白云母,包含结构不发育.斜长石以更长石为主,An平均值为22;黑云母为富镁黑云母,MF值为0.5120～0.7216;普遍见有角闪石,Mg# =0.48～1.00.两类成矿岩浆岩的岩石学和矿物学差异可以为南岭花岗质岩石地区找矿提供初步的指示意义.     

浅析花岗岩类成矿专属性判别

花岗岩类具有成矿专属性。在不同大地构造背景及造山作用不同阶段,不同源岩的部分熔融形成类型、矿物学、岩石学和地球化学成分、成因、分异演化均不同的花岗岩类,它们均与相关的成矿元素组合具有一定的相关系,对花岗岩类成矿起关键性的控制作用。由此可见在判别花岗岩类的成矿专属性时需要综合考虑:野外判别、岩石化学成分判别、地球化学特征判别、花岗岩类成矿元素组合的大地构造控制判别。     

Zircon ages and Hf isotopic compositions of plutonic rocks from the Central Tianshan (Xinjiang,northwest China) and their significance for early to mid-Palaeozoic crustal evolution

We present new zircon ages and Hf-in-zircon isotopic data for plutonic rocks and review the crustal evolution of the Chinese Central Tianshan (Xinjiang, northwest China) in the early to mid-Palaeozoic. The Early Ordovician (ca. 475–473 Ma) granitoid rocks have zircon ε_Hf(t) values either positive (+0.3 to +9.5) or negative (?6.0 to ?12.9). This suggests significant addition of juvenile material to, and coeval crustal reworking of, the pre-existing continental crust that is fingerprinted by numerous Precambrian zircon xenocrysts. The Late Ordovician–Silurian (ca. 458–425 Ma) rocks can be assigned to two sub-episodes of magmatism: zircon from rocks of an earlier event (ca. 458–442 Ma) has negative zircon ε_Hf(t) values (?6.3 to ?13.1), indicating a predominantly crustal source; zircon from later events (ca. 434–425 Ma) has positive zircon ε_Hf(t) values (+2.6 to +8.9) that reveal a predominantly juvenile magma source. The Early Devonian (ca. 410–404 Ma) rocks have near-zero zircon ε_Hf(t) values, either slightly negative or positive (?1.4 to +3.5), whereas the Mid-Devonian rocks (ca. 393 Ma) have negative values (?11.2 to ?14.8). The Late Devonian (ca. 368–361 Ma) granites are undeformed and are chemically similar to adakite but have relatively low negative whole-rock ε_Nd(t)values (?2.4 to ?5.3). We interpret the Early Ordovician to Mid-Devonian magmatic event to reflect combined juvenile crustal growth and crustal reworking processes via episodic mafic underplating and mantle–crust interaction. The Late Devonian episode may signify delamination of the over-thickened Chinese Central Tianshan crust.     

Franciscan mélanges: coherent blocks in a low-density,ductile matrix

ABSTRACT

The Franciscan Complex comprises the largely sedimentary basement of the California Coast Ranges. This classic trench deposit has undergone a series of superimposed tectonic events since the end of Jurassic time, involving accretion, high-pressure (HP) recrystallization, buoyancy and wedge-driven exhumation, and transcurrent slip. Processes reflect plate convergence, transpressive-orthogonal subduction, and transpressive–transtensive offset. Besides stratigraphically intact strata, the Franciscan displays widespread mélanges of four main types: diapiric serpentinite intrusions, sedimentary olistostromes, broken formations, and tectonic block-in-matrix units. In the northern Coast Ranges, mélanges are especially prevalent in the Central Belt, but also occur in the Eastern and Coastal belts. Diapirs show upward, buoyant flow relative to wall rocks, but some also appear to have involved wedge-driven thrusting. Many serpentinite diapirs and tectonic mélanges contain exotic metamafic inclusions rimmed by actinolite–chlorite reaction rinds. Olistostromes include gravity slump blocks and conglomeratic lenses; petrologically similar to larger slide blocks, pebble layers document a surficial, sedimentary origin, as does the presence of volcanic arc clasts. Broken formations grade by degrees from intact stratal continuity to disrupted units; they only contain cognate boudins of rocks present in the ductile matrix. Some tectonic mélanges are simply intensely disaggregated broken formations, and include rock types of the stratigraphic host. Other tectonic mélanges carry exotic HP blocks of diverse lithologies, generally reflecting higher pressures than attended recrystallization of the low-density matrix. The four mélange types formed through diverse convergent plate-tectonic processes. Many were subjected to a multi-stage overprint; most are strongly deformed, obscuring original textures and structures. Broken formations are the most common disrupted units, accompanied by lesser amounts of tectonic mélanges, olistostromes, and ductile-matrix diapirs. In aggregate, these units reflect the operation of contrasting processes that attest to plate-tectonic evolution of the Franciscan Complex. Strong deformation accompanied oceanic plate underflow, but also took place during coeval HP metamorphism and surfaceward return of accretionary packets, then transitioned to long-sustained, chiefly dextral slip.     

内蒙古二连浩特北部阿仁绍布地区晚石炭世花岗岩Sr-Yb分类及其成因

对分布在内蒙古二连浩特北部阿仁绍布地区的晚石炭世花岗岩类,依据Sr、Yb含量,划分为低Sr高Yb型、极低Sr高Yb型和低Sr低Yb型花岗岩3种类型。低Sr高Yb型花岗岩类相对低Si、富Al,Na2O〉K2O,稀土元素分馏中等,有或无负Eu异常,Sr含量低,平均为183×10-6,Ba含量较高,平均585×10-6,Y含量高,平均30.06×10-6,Rb/Sr比值较低,平均0.97;极低Sr高Yb型花岗岩富Si、REE,低Al、Sr、Ba,高的Rb/Sr比值（平均为7.47）,具明显的负Eu异常等;低Sr低Yb型花岗岩富Si,贫Al、Ca、Mg,重稀土元素（Y、Yb）含量低,Y含量在（7.26～10.6）×10-6之间,平均9.76×10-6,Yb含量在（1.04～1.89）×10-6之间,平均1.44×10-6,δEu=0.64～0.94,具弱负Eu异常,微量元素Ba含量高,Rb/Sr比值低。3种类型的花岗岩类过铝指数（A/CNK）多小于1.0,说明它们均源自变质火成岩的部分熔融。由于源区的深度不同（pT条件不同）和残留的主要矿物相不同,它们的岩石地球化学特征存在差异。极低Sr高Yb型花岗岩形成深度最浅（中上地壳）,熔融残留相以斜长石为主;低Sr高Yb型花岗岩类形成于中下地壳,熔融残留相为斜长石和辉石;低Sr低Yb型花岗岩形成深度最深,推测可能形成于加厚下地壳（〉40km）底部,熔融残留相为石榴子石、斜长石和角闪石。     

Structure of the Late Palaeozoic Coffs Harbour Beds,northeastern New South Wales

F₁ macroscopic folds in the Late Palaeozoic Coffs Harbour Beds in the SE portion of the New England Fold Belt are commonly transected by cleavage. These macroscopic folds are tight to isoclinal structures, with a consistent vergence to the NE. Axial surfaces are either steeply dipping to the SW or vertical, and are typically faulted. Anomalous bedding‐cleavage relations occur where the steeply dipping cleavage intersects overturned limbs of F₁ macroscopic and some F₁ mesoscopic folds. Elsewhere F₁ mesoscopic folds have a well developed, axial‐surface cleavage and are rarely downward facing. Cleavage is commonly strike‐divergent from axial surfaces of F₁ macroscopic folds, except adjacent to the Demon Fault System, where they are parallel. These anomalous cleavage‐folds relations possibly developed during the one deformation. D₁ structures are refolded by kink‐like folds that are steeply plunging. The structural style of the D₁ deformation indicates that it possibly resulted from accretionary processes at a consuming plate margin.