Granite emplacement mechanisms and tectonic controls: inferences from deformation studies

Abstract

This paper is a structural and tectonic approach to the emplacement and deformation of granitoids. The main methods available in structural geology are briefly reviewed and this emphasises that (a) a wealth of data, particularly strain and shear sense, which pertain to these problems, can be determined in and around plutons; (b) given the nature, unlike many other crustal rock types, of granites to crystallise from isotropic through weakly anisotropic crystal suspension fluids, that deformation which has occurred in these states may not be well preserved; and (c) it is entirely possible, using this methodology, to separate deformation resulting from externally originating tectonic stresses from that which is associated with internal magma-related stresses. It is also recommended that the genetically-based Cloosian classification of granite fabrics and structures into “primary” (magmatic flow/magmatic flow current) and “secondary”, be abandoned and that a more observationally-based approach which classifies granite deformation fabrics and structures according to their time of occurrence relative to the crystallisation state of the congealing magma, be adopted (i.e. pre-full crystallisation deformation and crystal plastic strain deformation).Examples of recent, structurally based, studies of emplacement mechanisms of plutons within tectonic settings are described and these show that, in general, space for magma can be created by the combination of tectonically-created cavities and internal magma-related buoyancy. This occurs in both transcurrent and extensional tectonic settings and there is no reason to doubt that it can happen in compressive-contractional regimes. It is concluded that transient and permanent space creation, such as may be exploited by available magmas, is a typical feature of the tectonically stressed and deforming lithosphere and this, in combination with the natural buoyancy and ascending tendency of magmas, can generate the varied emplacement mechanisms of granites.

Estruturas e histórias deformacionais contrastantes dos granitos sintectônicos de Campina Grande e Serra Redonda, Província Borborema, NE do Brasil

“…Numerosos estudos estruturais em plútons graníticos confirmam a forte conexão entre a trama (fabric) magmática e a deformação regional, especialmente em zonas de cisalhamento (e.g., Hutton, 1988;Vigneresse, 1995;Brown e Solar, 1998;Cruden, 2006 etc.). O estudo da relação entre o magmatismo e a deformação constitui uma das ferramentas essenciais para entender os mecanismos de transferência de massa e diferenciação da crosta e, assim, a própria evolução estrutural de um oró-geno.…”

Determination of Rock Cleavages Using AMS (Anisotropy of Magnetic Susceptibility): a Case Study on the Geochang Granite Stone, Korea

“…따라서 이들 자기엽리구조와 석재 결의 방향 성의 상관성을 규명할 수 있다면, 석재 결의 파악 및 채굴과 가공의 효율성 향상에 크게 기여할 것으로 판 단된다. 화강암에 발달하는 미세구조는 기원과 메카니즘에 따라 크게 4가지로 분류되어 왔다 (Balk, 1937;Barrire, 1981;Hutton, 1982Hutton, , 1988Abbott, 1989;Paterson et al, 1989;Vigneresse, 1990;Philpotts and Asher, 1994;Tobisch and Cruden, 1995) 한편, BW무역과 MD기업사의 자기미세구조와 석 재 결의 방향성 비교 결과는 앞서 기술한 석산의 결 과와는 뚜렷한 차이를 보인다 (Fig. 11e, f) …”

Magma emplacement in a thrust ramp anticline: The Gavorrano Granite (northern Apennines, Italy)

“…[53] In the proposed model, the absence of magmatic foliations in the Gavorrano Granite and emplacementrelated ductile deformation in the host rocks allow us to suggest that the Gavorrano Granite was emplaced as a permitted synkinematic intrusion [Hutton, 1988]. However, in order to explain the absence of magmatic foliations, the physical condition of emplaced magma has to be considered.…”