Academic literature on the topic 'Intrusions (Geology) – Musgrave Block (W.A.)'

AbstractAnalyses of structures in the western part of the North Patagonian Massif (southern Argentina) suggest a polyphase evolution, accompanied by continuous intrusive activity. The first two deformations (D1, D2) and metamorphism affected the upper Palaeozoic, partly possibly older Cushamen Formation clastic succession and different intrusive rocks. A second group of intrusions, emplaced after the second deformational episode (D2), in many places contain angular xenoliths of the foliated country rocks, indicating high intrusive levels with brittle fracturing of the crust. Deformation of these magmatic rocks presumably began during (the final stage of) cooling and continued under solid-state conditions. It probably coincided with the third deformational event (D3) in the country rocks. Based on published U–Pb zircon ages of deformed granitoids, the D2-deformation and younger event along with the regional metamorphism are likely to be Permian in age. An onset of the deformational and magmatic history during Carboniferous times, however, cannot be excluded. The estimated ~W–E to NE–SW compression during the D2-deformation, also affecting the first group of intrusive rocks, can be related to subduction beneath the western Patagonia margin or an advanced stage of collisional tectonics within extra-Andean Patagonia. The younger ~N–S to NE–SW compression might have been an effect of oblique subduction in the west and/or continuing collision-related deformation. As a cause for its deviating orientation, younger block rotations during strike-slip faulting cannot be excluded. The previous D2-event presumably also had an effect on compression at the northern Patagonia margin that was interpreted as result of Patagonia's late Palaeozoic collision with the southwestern Gondwana margin. With the recently proposed Carboniferous subduction and collision south of the North Patagonian Massif, the entire scenario might suggest that Patagonia consists of two different pieces that were amalgamated with southwestern Gondwana during Late Palaeozoic times.

Abstract. Thrust fault systems typically distribute shear strain preferentially into the hanging wall rather than the footwall. The Woodroffe Thrust in the Musgrave Block of central Australia is a regional-scale example that does not fit this model. It developed due to intracontinental shortening during the Petermann Orogeny (ca. 560–520 Ma) and is interpreted to be at least 600 km long in its E–W strike direction, with an approximate top-to-north minimum displacement of 60–100 km. The associated mylonite zone is most broadly developed in the footwall. The immediate hanging wall was only marginally involved in the mylonitization process, as can be demonstrated from the contrasting thorium signatures of mylonites derived from the upper amphibolite facies footwall and the granulite facies hanging wall protoliths. Thermal weakening cannot account for such an inverse deformation gradient, as syn-deformational P–T estimates for the Petermann Orogeny in the hanging wall and footwall from the same locality are very similar. The distribution of pseudotachylytes, which acted as preferred nucleation sites for shear deformation, also cannot provide an explanation, since these fault rocks are especially prevalent in the immediate hanging wall. The most likely reason for the inverted deformation gradient across the Woodroffe Thrust is water-assisted weakening due to the increased, but still limited, presence of aqueous fluids in the footwall. We also establish a qualitative increase in the abundance of fluids in the footwall along an approx. 60 km long section in the direction of thrusting, together with a slight decrease in the temperature of mylonitization (ca. 100 °C). These changes in ambient conditions are accompanied by a 6-fold decrease in thickness (from ca. 600 to 100 m) of the Woodroffe Thrust mylonitic zone.

Well-grouped stable magnetizations have been isolated at 14 of 20 sites sampled from the Spetch Creek pluton. The single-polarity primary magnetization directed at D = 026.2°, I = 73.5° (α95 = 4.4°, paleolatitude 59 ± 7°N, paleopole 73°N, 078°W, A95 = 7°) was acquired around 88 Ma during the Cretaceous normal polarity superchron (118–84 Ma). This direction is discordant from the expected mid-Cretaceous direction (D = 332.5°, I = 75.1°) for North America. The difference could be caused by one of two end member models: the all-tilt model requires a 15° east-side-up tilt about a horizontal axis striking 353°, and the displacement–rotation model requires 330 ± 770 km of northward displacement combined with 54 ± 14° of clockwise rotation. Regardless, this result provides a negative test of the Baja British Columbia model, which requires ~ 2400 km of northward displacement.A review of previously observed mid-Cretaceous magnetizations from the Coast Belt, which are also discordant, indicates that they exhibit common characteristics, although their discordance is not uniform. Assuming that present horizontal approximates paleohorizontal, post-mid-Cretaceous latitudinal displacements inferred from individual results vary between 330 and 3500 km northwards. Relative rotations about vertical axes vary between 17 and 57° clockwise. Such variation cannot be accounted for by the displacement and rotation of a superterrane as a whole. Recent studies emphasize that many of the intrusions have probably been locally tilted. Consistent with known geology, these discordant poles are best explained by the "tilt and moderate displacement" model. This model invokes moderate (500–1000 km) post-mid-Cretaceous northward displacement of the amalgamated Insular, Coast, and Intermontane terranes west of the major dextral Canadian Cordilleran fault systems, combined with variable local block tilting east- and northeast-side-up. Northward displacement was driven by Kula and (or) Farallon – North American plate interactions from 90 to 56 Ma. Tilting is most likely due to a combination of northeast–southwest compression, differential uplift, and extension.

The Nebo-Babel Ni-Cu-platinum-group element (PGE) magmatic sulphide deposit, a world-class ore body, is hosted in low-MgO, tube-like (chonolithic) gabbronorite intrusion in the West Musgrave Block, Western Australia. The Nebo-Babel deposit is the first significant discovery of a nickel sulphide deposit associated with the ca. 1078 Ma Giles Complex, which is part of the Warakurna large igneous province (LIP), now making the Musgrave Block a prime target for nickel sulphide exploration. The Musgrave Block is a Mesoproterozoic, east-west trending, orogenic belt in central Australia consisting of amphibolite and granulite facies basement gneisses with predominantly igneous protoliths. The basement lithologies have been intruded by mafic-ultramafic and felsic rocks; multiply deformed and metamorphosed between 1600 Ma and 500 Ma. The Giles Complex, which is part of the Warakurna LIP, was emplaced at ca. 1078 Ma and consists of a suite of layered mafic-ultramafic intrusions, mafic and felsic dykes and temporally associated volcanic rocks and granites. The Giles Complex intrusions are interpreted to have crystallised at crustal depths between 15km and 30km and are generally undeformed and unmetamorphosed.