Successive overprinting granulite facies metamorphic events in the Anmatjira Range, central Australia

The Anmatjira Range and adjacent Reynolds Range, central Australia, comprise early Proterozoic metasediments and othogneisses that were affected by three, and possibly four, temporally distinct metamorphic events, M_1–4, and deformation events, D_1–4, in the period 1820–1590 Ma. The north-western portion of the range, around Mt Stafford, preserves the effects of ±1820 Ma M₁-D₁, and shows a spectacular lateral transition from muscovite + quartz-bearing schists to interlayered andalusite-bearing migmatites and two-pyroxene granofelses that reflect extremely low-pressure granulite facies conditions, over a distance of less than 10 km. Orthopyroxene + cordierite + garnet + K-feldspar + quartz-bearing gneisses occur at the highest grade, implying peak conditions of ±750°C and 2.5 ± 0.6 kbar. An anticlockwise P–T path for M₁ is inferred from syn- to late-D₁ sillimanite overprinting andalusite, petrogenetic grid considerations and quantitative estimates of metamorphic conditions for inferred overprinting assemblages. The effects of M₁ have been variably overprinted to the south-east by a c. 1760 Ma M₂–D₂ event. Much of the central Anmatjira Range, around Ingellina Gap, comprises orthogneiss, deformed during D₂, and metapelites that have M₁ andalusite and K-feldspar overprinted by M₂ sillimanite and muscovite. The south-eastern portion of the range, around Mt Weldon, comprises metasediments and orthogneisses that were completely recrystallized during M₂–D₂, with metapelitic gneisses characterized by spinel + sillimanite + K-feldspar + quartz-bearing assemblages that suggest peak M₂ conditions of >750°C and 5.5 ± 1 kbar. Overprinting parageneses in metapelitic gneisses imply that D₂ occurred during essentially isobaric cooling. A third granulite facies event, M₃, affected rocks in the Reynolds Range, immediately to the south of the Anmatjira Range, at c. 1730 Ma. A possible fourth event, M₄, with a minimum age of c. 1590 My affected both Ranges, but resulted in only minor overprinting of M_1–3 assemblages. The superimposed effects of M_1–4, mapped for the entire Anmatjira–Reynolds Range area, indicate that only minor or no dislocation of the regional geology occurred during any of the metamorphic and accompanying folding, events. Although the immediate cause of each of the metamorphic events involved advection, the ultimate causes were external to the metasediments and most probably external to the crust.     

昌马地震应力场及发震机制研究

本文利用前人大量的地质、地球物理资料，建立了昌马盆地的立体地质模型，利用三维有限元法研究了１９３２年昌马７．６级地震前后应力场的变化及其昌马地震成因机制，结果表明：（１）震源断层走向与利用主压应力推测的断裂走向相差６５°，可能是由于地壳浅部旋转的主压应力与深部北东方向的主压应力所产生的力矩使块体发生旋转所致，其枢纽点即为震源所在；同时，该旋转也导致了地震断层和震源断层在力学性质和几何性质上的不同。（２）昌马地震是在闭锁、贯通和块体旋转联合作用下形成的。（３）区域应力场与局部应力场不一致、应力松弛单元的出现可能与地震的孕育、发生有关。（４）断裂活动的不均一性与平面最大剪应力分布不均匀成正相关，且各断裂均以相对左旋走滑兼挤压为特征，但是其走滑量的分布是不均匀的。     

Petrogenesis of the granite related to the Baishaziling Sn deposit,Dayishan ore field,Southern China

The Baishaziling greisen-type tin deposit is located in the Dayishan ore field, Nanling Range, Southern China. In this study, for the first time, we present both zircon and cassiterite UPb dating, whole-rock elements, zircon LuHf and apatite Nd isotopic compositions to better constrain the petrogenesis of granite and its genetic link with Sn mineralization. Zircon UPb ages of fine-grained granite and coarse-grained granite are 154 ± 1.8 Ma and 153 ± 2.1 Ma, respectively, which are consistent with the cassiterite UPb dating of 154 ± 5.4 Ma, implying genetic relationship between the Baishaziling granite and tin metallogenesis. The Baishaziling granites exhibit high SiO₂, K₂O + Na₂O, Zr + Nb + Ce + Y contents, low P₂O₅ and Sr contents, and high ratios of Ga/Al, ^TFeO/(^TFeO + MgO), implying A-type granite affinity with characteristics of high-K calc-alkaline and weakly peraluminous. The zircon ε_Hf(t) values and apatite ε_Nd(t) values of the granite vary from ?4.46 to ?1.81 and ?8.37 to ?7.10, with two-stage Hf and Nd model ages of 1.40 to 1.50 Ga and 1.52 to 1.64 Ga, indicating that they were generated by the partial melt of the Proterozoic basement with the involvement of mantle magma. In addition, formation of Dayishan granite was likely associated with an intraplate extensional setting caused by the subduction of the Palaeo-Pacific plate. The Baishaziling reduced granites have high stannum and boron contents, which are in favor of the tin mineralization.     

佳木斯地块构造演化

佳木斯地块位于中亚造山带东段,是我国东北地区一个重要的大地构造单元,古生代以来经历了复杂的多构造体系叠合的演化过程。本文在总结近二十年已报导的相关研究成果基础上,结合笔者近年工作,探讨了佳木斯地块的基底属性和来源,重塑了佳木斯地块西缘碰撞拼贴,以及东缘俯冲-增生的构造演化过程。研究表明,佳木斯地块具有亲冈瓦纳大陆的构造属性,裂离后经历了长距离的北漂。与松辽地块先后两次拼合,首次发生于中志留世(～425Ma),在晚二叠世前后(～250Ma)沿原缝合带位置发生裂解,拉张出新的有限洋盆(牡丹江洋),并于侏罗纪(185～145Ma)与松辽地块沿牡丹江-依兰构造带再次碰撞拼贴,形成了高压变质的黑龙江增生杂岩带。而佳木斯地块东缘受晚石炭世-晚三叠世(305～250Ma)泛大洋的俯冲-增生事件影响,形成了跃进山增生杂岩,随后于中侏罗世-早白垩世(165～128Ma)在古太平洋板块的西向俯冲作用下,形成了饶河增生杂岩。因此,佳木斯地块的构造演化既涉及了晚古生代古亚洲洋构造域的消亡,又经历了中生代古太平洋构造域的叠加与改造,而黑龙江杂岩的形成标志着古太平洋构造体制与古亚洲洋构造体制的转换始于晚三叠世(～210Ma)。     

NEW ACTIVITY CHARACTERISTICS AND SLIP RATE OF THE EBOMIAO FAULT IN THE SOUTHERN MARGIN OF BEISHAN,GANSU PROVINCE

The Ebomiao Fault is a newly discovered active fault near the block boundary between the Tibetan plateau and the Alashan Block. This fault locates in the southern margin of the Beishan Mountain, which is generally considered to be a tectonically inactive zone, and active fault and earthquake are never expected to emerge, so the discovery of this active fault challenges the traditional thoughts. As a result, studying the new activity of this fault would shed new light on the neotectonic evolution of the Beishan Mountain and tectonic interaction effects between the Tibetan plateau and the Alashan Block. Based on some mature and traditional research methods of active tectonics such as satellite image interpretation, trenches excavation, differential GPS measurement, Unmanned Aircraft Vehicle Photogrammetry(UAVP), and Optical Stimulated Luminescence(OSL)dating, we quantitatively study the new activity features of the Ebomiao Fault.
Through this study, we complete the fault geometry of the Ebomiao Fault and extend the fault eastward by 25km on the basis of the 20km-fault trace identified previously, the total length of the fault is extened to 45km, which is capable of generating magnitude 7 earthquake calculated from the empirical relationships between earthquake magnitude and fault length. The Ebomiao Fault is manifested as several segments of linear scarps on the land surface, the scarps are characterized by poor continuity because of seasonal flood erosion. Linear scarps are either north- or south-facing scarps that emerge intermittently. Fourteen differential GPS profiles show that the height of the north-facing scarps ranges from (0.22±0.02)m to (1.32±0.1)m, and seven differential GPS profiles show the height of south-facing scarps ranging from (0.33±0.1)m to (0.64±0.1)m. To clarify the causes of the linear scarps with opposite-facing directions, we dug seven trenches across these scarps, the trench profiles show that the south-dipping reverse faults dominate the north-facing scarps, the dipping angles range from 23° to 86°. However, the south-facing scarps are controlled by south-dipping normal faults with dipping angles spanning from 60° to 81°.
The Ebomiao Fault is dominated by left-lateral strike-slip activity, with a small amount of vertical-slip component. From the submeter-resolution digital elevation models(DEM)constructed by UAVP, the measured left-lateral displacement of 19 gullies in the western segment of the Ebomiao Fault are(3.8±0.5)~(105±25)m, while the height of the north-facing scarps on this segment are(0.22±0.02)~(1.32±0.10)m(L₃-L₇), the left-lateral displacement is much larger than the scarp height. In this segment, there are three gullies preserving typical left-lateral offsets, one gully among them preserves two levels of alluvial terraces, the terrace riser between the upper terrace and the lower terrace is clear and shows horizontal offset. Based on high-resolution DEM interpretation and displacement restoration by LaDiCaoz software, the left-lateral displacement of the terrace riser is measured to be(16.7±0.5)m. The formation time of the terrace riser is approximated by the OSL age of the upper terrace, which is (11.2±1.5)ka BP at (0.68±0.03)m beneath the surface, and(11.4±0.6)ka at (0.89±0.03)m beneath the surface, the OSL age (11.2±1.5)ka BP at (0.68±0.03)m beneath the surface is more close to the formation time of the upper terrace because of a nearer distance to sediment contact between alluvial fan and eolian sand silt. Taking the (16.7±0.5)m left-lateral displacement of the terrace riser and the upper terrace age (11.2±1.5)ka, we calculate a left-lateral strike-slip rate of(1.52±0.25)mm/a for the Ebomiao Fault. The main source for the slip rate error is that the terrace risers on both walls of the fault are not definitely corresponded. The north wall of the fault is covered by eolian sand, we can only presume the location of terrace riser by geomorphic analysis. In addition, the samples used to calculate slip rate before were collected from the aeolian sand deposits on the north side of the fault, they are not sediments of the fan terraces, so they could not accurately define the formation age of the upper terrace. This study dates the upper terrace directly on the south wall of the fault.
Since the late Cenozoic, the new activity of the Ebomiao Fault may have responded to the shear component of the relative movement between the Tibetan plateau and the Alashan Block under the macroscopic geological background of the northeastern-expanding of the Tibetan plateau. The north-facing fault scarps are dominated by south-dipping low-angle reverse faults, the emergence of this kind of faults(faults overthrusting from the Jinta Basin to the Beishan Mountain)suggests the far-field effect of block convergence between Tibetan plateau and Alashan Block, which results in the relative compression and crustal shortening. As for whether the Ebomiao Fault and Qilianshan thrust system are connected in the deep, more work is needed.     

鄂尔多斯地块南缘奥陶纪前陆盆地的沉积大地构造格架与演化

鄂尔多斯地块南缘位于鄂尔多斯地块与北秦岭造山带结合部位,是研究北秦岭造山带奥陶纪弧-陆碰撞事件沉积-构造响应的理想场所。目前对鄂尔多斯地块南缘奥陶纪盆地的沉积充填特征、构造属性、结构以及演化历史仍缺乏清晰的认识。本文通过对露头区典型剖面观测结果和覆盖区地球物理资料的综合分析,发现鄂尔多斯地块南缘奥陶纪盆地沉积充填整体表现为南厚北薄、顶底均被区域性不整合面所限定的楔状沉积体;由北向南可以划分出局限沉积环境的坳陷带、持续抬升的隆起带和局限浅水-海相深水-陆相冲积扇环境的盆地带。结合北秦岭造山带奥陶纪大地构造属性及演化历史分析,得出鄂尔多斯地块南缘奥陶纪盆地是与北秦岭造山带奥陶纪弧-陆碰撞事件相关的周缘前陆盆地,经历了以下四个阶段的演化:(1)冶里-亮甲山期,前陆盆地初始挠曲阶段;(2)马一-马五期,前陆盆地结构成型阶段;(3)马六-平凉期,前陆盆地快速沉降阶段;(4)背锅山期,前陆盆地快速充填阶段。此外,该周缘前陆盆地具有经典前陆盆地的四元结构:褶皱冲断带位于包括渭河地堑在内的西安-宝鸡一带,前渊带呈近东西向展布于岐山-富平一带,前缘隆起带位于庆阳-运城一带,隆后坳陷带大致位于延安-榆林-绥德一带,呈不规则的椭圆形。该研究不仅有助于查明鄂尔多斯地块南缘奥陶纪盆地性质、结构与演化历史,还为北秦岭造山带奥陶纪弧-陆碰撞事件提供了盆地内部的地质历史证据。     

Mineral distribution and provenance of heavy mineral sands (zircon,ilmenite, rutile) deposits from the NW Murray Basin,far western NSW,Australia

Abstract

There is significant economic interest in the Murray Basin of southeastern Australia as it is proving to be a major heavy mineral sands (HMS) province that will be one of Australia’s major source for production of rutile, zircon and ilmenite. The distribution and provenance of HMS resources in the Murray Basin is poorly understood because of its huge size, limited exploration and the complex depositional, structural and weathering mechanisms in their development. In this paper, we focus on the Copi North and Magic deposits some 130–180?km south of Broken Hill, NSW. The heavy mineral assemblages of the Copi North and Magic deposits are very similar, with the main economic minerals being ilmenite, leucoxene, pseudorutile, rutile and zircon. Intensive fracturing and brecciation are identified in many samples and are inferred to have been initially caused by multi-stage deformational processes associated with metamorphism and then further developed through alluvial and eolian transportation. Both deposits are classified as ‘medium sands, symmetrical, mesokurtic and moderately well-sorted’. The majority of economic minerals are of low to medium sphericity and subrounded, along with abundant polished eolian quartz grains. The Copi North deposit has coarser and more poorly sorted sediments with higher HMS grades and magnetics content than the Magic deposit, reflecting a higher energy depositional environment. The main source for the HMS for the Copi North and Magic deposits is largely ascribed to the Broken Hill Block. Previous studies have shown that the Broken Hill orebody underwent substantial sub-aerial weathering over hundreds of millions of years. In addition, the complex metamorphic events experienced by the Broken Hill Block were capable of forming the broad series of minerals identified within the Copi North and Magic deposits. The HMS were believed to have been transported through paleovalleys near the Mulculca Fault in a southeast direction representing a feeder system into the NW Murray Basin. Both deposits feature a relatively linear geometry (roughly parallel to the strike of the paleoshoreline), with high HMS grades, modest tonnages, and coarser sediments when compared to WIM-style offshore deposits. Compared to other strandline HMS deposits of the Murray Basin, they are smaller in size although have similar high grades of 3.7–6.9% THM and similar proportions of the HMS assemblage of ilmenite, leucoxene, rutile and zircon. Deposits of similar size and grade are likely to occur throughout the northern part of the Murray Basin.

1. KEY POINTS

2. Both the Copi North and Magic deposits contain similar mineral assemblages with the provenance of the heavy minerals ascribed to the Broken Hill Block.

3. A relatively high energy inshore environment is inferred for the Copi North deposit while a lower energy environment associated with either a foreshore or backshore environment is inferred for the Magic deposit.

4. Deposits of similar mineralogy, grades and size are likely to occur elsewhere throughout the northern Murray Basin.

     

In-situ migmatite and hybrid diatexite at Mt Stafford, central Australia

Metasedimentary gneisses show a rapid change in grade within a 10-km-wide low- P /high- T  regional aureole at Mt Stafford, Arunta Block, central Australia. Migmatites occur in all but the lowermost of five metamorphic zones, which are characterized by: (1) muscovite–quartz schist; (2) andalusite–cordierite–K-feldspar granofels with small melt segregations; (3) spinel–sillimanite–cordierite–K-feldspar migmatite; (4) garnet–orthopyroxene–cordierite migmatite and minor diatexite; and (5) biotite–cordierite–plagioclase diatexite that shows a transition to granite. A subsolidus unit comprising interbedded sandstone and siltstone is equivalent to bedded migmatite , the main rock type in Zones 2–4. Mesoscopic textures and migmatite classification of this unit vary with grade. In Zone 2, metatexite is developed in siltstone layers that are separated by quartz-rich, unmelted metapsammite layers. Melt segregation was less efficient in Zones 3 and 4, where the dominant migmatite layering is a modified bedding. High proportions of melt were present in Zone 4, in which schlieren migmatite is transitional between bedded migmatite and metapelite-sourced diatexite. The preservation of sedimentary structures and coexistence of melt reactants and products in Zone 4 metapelite imply that melting proceeded in situ without substantial migration of melt. Zone 5 biotite–cordierite–plagioclase diatexite carries rafts of bedded migmatite with strongly resorbed edges, as well as large K-feldspar and quartz augen. This unit of comparatively Ca-rich migmatites is inferred to have been formed by the mixing of locally derived and injected granitic melt.     

中国满洲里──绥芬河地学断面地球物理综合研究

叙述满洲里－绥芬河地学断面的地球物理成果．断面全长１３００ｋｍ，穿越了中国东北的大兴安岭岩浆岩带和张广才岭岩浆岩带，松辽盆地和海拉尔盆地，以及郯庐断裂北延部分的敦化－密山断裂、佳木斯－伊通断裂．结合地质资料，解释了断面多种地球物理方法的资料，包括重力、磁力、大地电磁（ＭＴ）、地震（广角地震测深、垂直反射地震与天然地震）、地热、古地磁以及综合解释，给出了断面岩石层构造的地球物理解释模型．讨论了松辽盆地基底的性质、莫霍界面的性质、大兴安岭重力梯级带的特征与成因，以及断面域地块拼合过程．研究结果认为，东北亚大地是一个拼合的增生大陆，是世界上一种新型的小地块碰撞拼合时受大洋板块俯冲的应力场作用形成的盆、山相间结构．     

华南早前寒武纪陆壳基底及其组成特征

在编制《闽浙赣铀矿地质图（１：１００万）》的基础上，结合近年来地质、地球物理及同位素年代学等方面的最新研究成果，本文初步分析了华南地区早前寒武纪（Ａｒ＿２－Ｐｔ＿１）陆壳结晶基底的组成特征及“扬子”与“华夏”地块之间的相互关系，认为该基底经吕梁运动（１７００－１８００Ｍａ）后，于中晚元古代逐渐解体，并转化为由板块构造机制所控制的多旋回槽台发展体制。由于古陆壳组成的不均一性和演化的不平衡性，使局部地区相对富铀，为尔后各种衍生型富铀地质体的形成及矿化分布奠定了基础。