Thông tin tài liệu
236 IGNEOUS ROCKS/Granite whereas economic deposits of tin, tungsten, and tantalum are related to ilmenite-bearing intrusives Where are Granites Found? Granitic rocks, their extrusive equivalents (rhyolites and dacites), and sedimentary and metamorphic rocks of granitic composition are easily the most abundant constituents of the upper continental crust, and they are widespread on all continents An early observation was that large composite granitic bodies (batholiths) were commonly, but not exclusively, found as linear belts defining mountain ranges and, therefore, that their emplacement was linked in some way to orogenesis or at least to large-scale interactions between lithospheric plates (see Tectonics: Mountain Building and Orogeny) The paucity of granitic bodies in oceanic environments is testimony to the importance of continental materials, and possibly lithospheric thickness, in the genesis of these magmas, implying that granitic generation is associated with the remobilization of older crust However, granitic bodies are encountered in all tectonic settings, although their volumes, compositions, and the nature of the associated igneous rocks vary accordingly (Table 2) The relationship between granitic composition and tectonic setting can be used to reconstruct the tectonic evolution of ancient orogenic belts Ascent and Emplacement Mechanisms Granitic bodies range in volume from less than km3, for single intrusions, to in excess of 106 km3, for example in batholithic structures such as the Coast Plutonic Complex in British Columbia They are classically depicted as circular-to-elliptical plutons on geological maps, though the shapes, field relationships, and internal structures of these bodies may be far more complex The outcrop pattern of a granite pluton depends on the level and topography of exposure and on the three-dimensional shape of the body To a large extent, the latter is dictated by the emplacement mechanism, which is in turn related to the depth of emplacement – and thus to the rheology of the host rocks – and to the geodynamic regime into which the pluton was intruded Emplacement occurs at a neutral buoyancy level or mechanical discontinuity, such as the brittle–ductile transition, whereupon magma flow switches from dominantly vertical to dominantly horizontal The way in which large plutonic masses are emplaced in the rigid upper crust has long been contentious The so-called ‘room problem’ has even formed the basis of arguments that granite has a non-magmatic origin However, our concepts of how large volumes of granitic magma are extracted from the source region, transported tens of kilometres through the crust, and intruded at higher structural levels have recently been revolutionized in two ways First, the traditional view that partial melting in the deep crust proceeds until a rheological threshold is reached, whereupon the source region slowly upwells en masse as a gravity-driven hot-Stokes diapir has been largely supplanted by a more dynamic scenario, in which the entire magma segregation, extraction, and transport process is controlled by deviatoric stress attending tectonic activity, perhaps augmented by compaction at high melt percentages The exact way in which this occurs is probably specific to the source rock type and the nature and rate of deformation, but valuable insight is provided by deep-crustal migmatite terranes These areas are characterized by deformation-enhanced segregation of partial melts from their residues and subsequent migration along planar anisotropies (foliations or lithological layering) to dilational structural sites, such as boudin-necks or fold axial surfaces (Figure 4A) An additional driving force is the volume increase associated with some partial-melting reactions, which can increase permeability by promoting microfracturing In areas of local melt accumulation, high magma pressures may develop in response to compressional stress and/or fluctuation in magma supply Subsequent magma (and thus heat) transfer to higher crustal levels occurs through interconnected networks of narrow structurally controlled channels or active shear zones during transpression, and as a result partial melts can be efficiently expelled from the source region Excellent examples of these relationships occur in the Arunta Inlier of central Australia and in the northern Appalachians of eastern North America (see North America: Northern Appalachians), where plutons are localized in regional shear-zone systems and rooted in migmatites In addition to its pervasive flow in distributed pathways, granitic magma may ascend from source regions in a more focused fashion by exploiting pre-existing faults, or as self-propagating dykes, essentially driven by buoyancy Thermal models have established that such magmatic conduits penetrating cold crust must maintain a critical thickness (typically 3–12 m) and magma-flow rate to avoid freezing and so are most viable where the generation and extraction of melt from the source is rapid Magmatic diapirism operating over short length scales may be of limited importance in the ductile deep crust, where thermal and rheological contrasts with the host rocks are minimal, and where the viscosity of granitic
Ngày đăng: 26/10/2022, 11:37
Xem thêm:
