From a detailed survey and sampling study of corrugated massifs north of the Fifteen-Twenty Fracture Zone on the Mid-Atlantic Ridge, we demonstrate that their surfaces are low-angle detachment fault planes, as proposed but not previously verified. Spreadingdirection-parallel striations on the massifs occur at wavelengths from kilometers to centimeters. Oriented drill-core samples from the striated surfaces are dominated by fault rocks with low-angle shear planes and highly deformed greenschist facies assemblages that include talc, chlorite, tremolite, and serpentine. Deformation was very localized and occurred in the brittle regime; no evidence is seen for ductile deformation of the footwall. Synkinematic emplacement of diabase dikes into the fault zone from an immediately subjacent gabbro pluton implies that the detachment must have been active as a low-angle fault surface at very shallow levels directly beneath the ridge axis. Strain localization occurred in response to the weakening of a range of hydrous secondary minerals at a very early stage and was highly efficient.
This contribution presents a new model for the Grampian-age tectonothermal development of the Buchan Block and Barrovian-type regions to its west, in the Grampian Terrane, Scotland. The model has drawn on evidence gathered from field mapping, microstructural analysis, metamorphic petrology and mafic magma geochemistry to propose that emplacement of the Grampian gabbros and regional metamorphic heating associated with production of Barrovian- and Buchan-type units occurred during syn-orogenic (Grampian-age), lithospheric-scale extension. Extension followed lithospheric thickening associated with the initiation of Grampian orogenesis and was followed by renewed lithospheric thickening and termination of the extensional heating. Mantle melting to produce the Grampian gabbros of the Grampian Terrane was achieved by extensional thinning of the lithosphere and decompression melting of the asthenosphere at depths of less than 70 km. Advection of heat from the mantle with emplacement of the Grampian gabbros augmented elevated heat budgets associated with attenuation of isotherms during extension. Deposition of the uppermost Dalradian (the Whitehills and Boyndie Bay Groups and the Macduff Slates) occurred during Grampian-age lithospheric extension. A gently-dipping, mid-crustal detachment focused metamorphic heat sources and accommodated significant lithospheric-scale strain, allowing independent thermal evolution of units in its hanging wall (the Buchan Block) and footwall (Barrovian-type units).
Security of supply of "hi-tech" raw materials (including the rare earth elements (REE) and some high-field-strength elements (HFSEs)) is a concern for the European Union. Exploration and research projects mostly focus on deposit-to outcrop-scale description of carbonatite-and alkaline igneous-associated REE-HFSE mineralization. The REE-HFSE mineral system concept and approach are at a nascent stage, so developed further here. However, before applying the mineral system approach to a chosen REE-HFSE metallogenic province its mineral system extent first needs defining and mapping. This shifts a mineral system project's foundation from the mineral system concept to a province's mineral system extent. The mapped extent is required to investigate systematically the pathways and potential trap locations along which the REE-HFSE mass may be distributed. A workflow is presented to standardize the 4-D definition of a REE-HFSE mineral system at province-scale: (a) Identify and hierarchically organize a mineral system's genetically related sub-divisions and deposits, (b) map its known and possible maximum extents, (c) name it, (d) discern its size (known mineral endowment), and (e) assess the favorability of the critical components to prioritize further investigations. The workflow is designed to generate process-based perspective and improve predictive targeting effectiveness along under-evaluated plays of any mineral system, for the future risking, comparing and ranking of REE-HFSE provinces and plays.or geophysical pathfinder signatures [7]; or serendipitously, e.g., during uranium prospecting [7]. Knowledge of the location controls of these metals remains limited compared to knowledge about base and precious metal deposits [5]. Many of the >500 known carbonatite occurrences [8] remain under evaluated. The result is recent REE mining from only a few deposits related to carbonatite and alkaline igneous bodies [1,9], e.g., the Bayan Obo Fe-REE deposit in China [10,11]; the Mountain Pass carbonatite-hosted REE deposit in the USA [12]; the Mount Weld carbonatite-associated lateritic REE and Nb-Ta deposits in Australia (e.g., Reference [13]) and the Araxa carbonatite-associated, lateritic Nb deposit in Brazil [14]. Other REE mines are of ion-adsorption clay deposits and monazitexenotime-bearing placer deposits (e.g., Reference [9]). For details of REE and carbonatite-related data, uses, occurrences, and mines refer to References [1,5,[15][16][17][18][19]. Aims and AudienceAs most REE-HFSE-centric exploration and research projects tend to focus efforts on the depositto outcrop-scale they neglect to place occurrences within their wider context, where significant undiscovered potential may exist. To help address this knowledge gap we present a method and workflow for defining, mapping and naming any REE-HFSE mineral system associated with carbonatite and alkaline igneous rocks. The objective is to 4-D summarise the current knowledge status of the selected mineral system. It is designed so that geoscientists, economists, and stra...
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