Delamerian Glenelg tectonic zone,western Victoria: Geology and metamorphism of stratiform rocks

The Cambro‐Ordovician Glenelg tectonic zone of western Victoria is a distinctive metamorphic‐igneous segment of the Delamerian Orogenic Belt comprising two northwest‐striking regional metamorphic segments of andalusite‐sillimanite type prograding towards an axial granitic batholith. The second of five deformations (D₂) was most significant, producing isoclinal folds, transposition and a pervasive regional foliation (S₂). Southwest of the central batholith, biotite to migmatite zones contain mainly quartzo‐feldspathic rock (turbiditic metagreywacke, quartzo‐feldspathic schist and migmatite), plus less common metaquartzite and calc‐silicate rocks and minor metapelite. Metagabbro, metadolerite and amphibolite typically have the chemistry of mid‐ocean ridge basalts. Serpentinite pods and sheets were tectonically introduced to low‐grade areas. Northeast of the central batholith, quartzo‐feldspathic rock occupies the sillimanite and migmatite zones exclusively, with a regional concentration of pegmatites adjacent to the zone boundary. Gross interleaving of quartzo‐feldspathic schist, migmatite, pegmatite and muscovite‐bearing granitic rock is characteristic. Peak metamorphic conditions of 550 MPa at 640°C leading to migmatite formation were established by D₂ time and accompanied by tonalite‐granodiorite and pegmatite emplacement. Subsequently, the thermal high contracted to the northeast culminating in the more extensive syn‐, post‐D4 to pre‐D5 granitic magmatism.     

Tower Hill gold deposit,Western Australia: An atypical,multiply deformed Archaean gold‐quartz vein deposit

The Tower Hill gold deposit is distinguished from most Archaean lode deposits of the Yilgarn Craton by virtue of its formation early in the regional deformation history and its consequent deformation. The deposit is located in ultramafic schist, adjacent to the contact with a small pluton of biotite monzogranite that intrudes pervasively foliated granodiorite, the dominant component of the Raeside Batholith. Gold, accompanied by local concentrations of bismuth minerals and molybdenite, occurs in a number of quartz vein ‘packages‘. Mineralised quartz veins at Tower Hill lie within an envelope of potassic alteration (talc‐biotite‐chlorite‐pyrite schist), up to several hundred metres wide. They are spatially and temporally associated with the biotite monzogranite and felsic porphyry intrusions, and their deformed equivalents. The deposit lies in a broad zone of ductile deformation (the Sons of Gwalia Shear Zone). Within the altered ultramafic schist, thin units of felsic schist, derived from biotite monzogranite and felsic porphyry, provided sites of contrasting competency that localised quartz vein formation. The mineralised quartz veins were subsequently deformed during alternating periods of shortening and extension, probably related to the syntectonic, solid‐state emplacement of the Raeside Batholith. These deformations pre‐dated strike‐slip movement on the Cemetery Fault, which truncates the ductile fabrics of the Sons of Gwalia Shear Zone, south of Tower Hill. In terms of the regional deformation history, gold mineralisation at Tower Hill formed during early D₂ (regional upright folding); subsequent deformation of the orebody pre‐dated D₃ (strike‐slip movement on the Cemetery Fault). The nearby Sons of Gwalia and Harbour Lights deposits also probably formed at an early stage, in contrast to most lode gold deposits in the Yilgarn Craton, which formed during or after D₃.     

Alice Springs age shear zones from the southeastern Reynolds Range,central Australia

The southeast Reynolds Range, central Australia, is cut by steep northwest‐trending shear zones that are up to hundreds of metres wide and several kilometres long. Amphibolite‐facies shear zones cut metapelites, while greenschist‐facies shear zones cut metagranites. Rb–Sr and ⁴⁰Ar–³⁹Ar data suggest that both sets of shear zones formed in the 400–300 Ma Alice Springs Orogeny, with the sheared granites yielding well‐constrained ⁴⁰Ar–³⁹Ar ages of ca 334 Ma. These data imply that the shear zones represent a distinct tectonic episode in this terrain, and were not formed during cooling from the ca 1.6 Ga regional metamorphism. A general correlation between regional metamorphic grade and the grade of Alice Springs structures implies a similar distribution of heat sources for the two events. This may be most consistent with both phases of metamorphism being caused by the burial of anomalously radiogenic heat‐producing granites. The sheared rocks commonly have undergone metasomatism implying that the shear zones were conduits of fluid flow during Alice Springs times.     

An example of structural and metamorphic relationships in the Musgrave orogenic belt,central Australia

The western Musgrave Ranges are broadly divided into three groups of metamorphic rocks. A central granulite‐facies core is bounded on the north by rocks of amphibolite grade and on the south by rocks transitional between the granulite and amphibolite facies. Faults trending east‐west separate the three groups of rocks.

The detailed structural relationships between the granulites and the lower grade rocks are described and discussed. The granulites are structurally relatively simple and are characterised by the presence of a strong southwesterly‐plunging, mineral‐streaking lineation. In marked contrast, the transitional rocks are more complexly folded on a macroscopic scale and they also have a well‐developed mineral lineation plunging to the southeast. These two lineation orientations are considered to be directions of maximum elongation. The amphibolite‐facies rocks are also complexly folded and at least two lineations related to different phases of deformation have been recognized.

A suite of foliated and lineated dolerite dykes which occurs throughout the area inherited their fabric during a period of intense deformation and recrystallization, which resulted in the development of numerous mylonite zones.

It is suggested that the granulite‐facies rocks may represent a suite of cover rocks which have been thrust in a northerly direction over a pre‐existing amphibolite‐facies basement.     

Woodleigh,Southern Carnarvon Basin,Western Australia: history of discovery,Late Devonian age,and geophysical and morphometric evidence for a 120 km-diameter impact structure

The discovery of the Woodleigh impact structure, first identified by R. P. Iasky, bears a number of parallels with that of the Chicxulub impact structure of K?–?T boundary age, underpinning complications inherent in the study of buried impact structures by geophysical techniques and drilling. Questions raised in connection with the diameter of the Woodleigh impact structure reflect uncertainties in criteria used to define original crater sizes in eroded and buried impact structures as well as limits on the geological controls at Woodleigh. The truncation of the regional Ajana?–?Wandagee gravity ridges by the outer aureole of the Woodleigh structure, a superposed arcuate magnetic anomaly along the eastern part of the structure, seismic-reflection data indicating a central >?37 km-diameter dome, correlation of fault patterns between Woodleigh and less-deeply eroded impact structures (Ries crater, Chesapeake Bay), and morphometric estimates all indicate a final diameter of 120 km. At Woodleigh, pre-hydrothermal shock-induced melting and diaplectic transformations are heavily masked by pervasive alteration of the shocked gneisses to montmorillonite-dominated clays, accounting for the high MgO and low K₂O of cryptocrystalline components. The possible contamination of sub-crater levels of the Woodleigh impact structure by meteoritic components, suggested by high Ni, Co, Cr, Ni/Co and Ni/Cr ratios, requires further siderophile element analyses of vein materials. Although stratigraphic age constraints on the impact event are broad (post-Middle Devonian to pre-Early Jurassic) high-temperature (200?–?250°C) pervasive hydrothermal activity dated by K?–?Ar isotopes of illite?–?smectite indicates an age of 359?±?4 Ma. To date neither Late Devonian crater fill, nor impact ejecta fallout units have been identified, although metallic meteoritic ablation spherules of a similar age have been found in the Canning Basin.     

辽宁弓长岭三星铁矿矿床地质特征及成因

弓长岭三星铁矿产于鞍山群茨沟组变质岩中,处于弓长岭背斜的北翼.区内主要有2层矿体,呈扁豆状,被F1断层切割成南北2个矿段,围岩主要为混合花岗岩.矿体矿石矿物主要为磁铁矿,矿石品级属需选贫矿石.根据区域铁矿资料,其成因类型为沉积变质型“鞍山式”贫铁矿.     

Eo-Alpine HP metamorphism in the Permian intrusives from the steep belt of the central Alps (Languard-Campo nappe and Tonale Series)

Abstract

Diorites and granitoids that intruded the Upper Austroalpine units of the central Alps during the Permian display map-pable tectonic imprints and metamorphic transformations that were acquired during the Alpine tectonometamorphic cycle. Superposed heterogeneous deformations interacted with metamorphic re-equilibration stages and created a range of textural types that reflect local deformation gradients: coronitic transformations textures, normally foliated S-tectonites and mylonitic foliations. The three textural types are distinguished on maps recording foliation trajectories of successive deformation phases, which are correlated to the evolution of metamorphic assemblages. Tectonic deformation of Alpine age is represented by three generations of ductile syn-metamorphic structures. The mineral assemblages stable during the first Alpine deformation phase (D1) are Amp_II + P1_II + white mica, + Zo/Czo + Grt + Qtz ± Mg-Ch1 ± Ilm in metadiorites and P1_II + white mica_I + Zo/Czo + Grt + Amp_II + Qtz ± Ilm/Ttn in metagranitoids; the successive foliations D2a and D2b are defined by greenschist facies minerals. Thermobarometric estimates allow T = 500–600 °C and P = 1.1 ± 0.2 GPa conditions to be determined during D1 and T ≤ 350 °C and P ≤ 0.5 GPa during D2. Relict igneous minerals in metadiorites allow to determine intrusive conditions of T = 879 ± 110 °C and P = 0.4–0.7 GPa. Radiometric ages and P/T ratio of Alpine P_maxT_Pmax suggest that the inferred P-T-d-t path may represent the thermal state of the initial Alpine subduction stages. © 2000 Éditions scientifiques et médicales Elsevier SAS