A hybrid origin for Lachlan S-type granites: the Murrumbidgee Batholith example
A hybrid origin for Lachlan S-type granites: the Murrumbidgee Batholith example
复制标题
DOI：
10.1016/j.lithos.2004.04.047
复制
发表时间：
2004-10
期刊：
影响因子：
3.5
通讯作者：
B. Healy;W. Collins;S. Richards
中科院分区：
文献类型：
--
作者：
B. Healy;W. Collins;S. Richards
Murrumbidgee花岗岩是澳大利亚东南部拉克伦褶皱带（LFB）其他复合S型花岗岩基的典型代表，由离散的过铝质同生花岗岩套组成。然而，它包含一个独特的套件（Murrumbucka）在其南端，具有化学和岩石学特征过渡与metaluminous I型花岗岩。以前详细的结构和变质研究表明，岩基向北倾斜，暴露了北部的次火山岩体和南部的根区，最初位于10公里深处。从南部的镁铁质、面理化、片状花岗岩到北部的长英质、面理化程度一般较低的均质花岗岩的过渡与岩浆上升通过近垂直的、结构控制的岩片在上覆岩浆房中侵位是一致的。在推断的根区，Murrumbucka套房主机混合变质沉积岩和辉长岩，这两者都有过渡接触，并显示证据的镁铁质-长英质相互作用与Murrumbucka套房。混合岩向南延伸，成为高T、低P的库马变质杂岩的一部分，该杂岩包含一个异质再活化闪长花岗岩（库马套）的核心，其透镜体也出现在整个Murrumbidgee岩石的南部（较深）部分。基岩中大多数镁铁质岩石的成分几乎所有元素都处于库马套花岗岩和辉长岩之间的化学联系线上。这种化学巧合被解释为反映派生的父母Murrumbidgee S型花岗岩岩浆之间的长英质（地壳）和镁铁质（幔源）成分，一致的Sr和Nd同位素结果和现场观测散装混合。基于连接线和简单的数值模拟，Murrumbucka套件估计是一个50：50的混合地幔和地壳衍生的岩浆，而更过铝质和广泛的清除范围套件，这是非常典型的拉克伦S型花岗岩，模拟为40：60的混合。化学变化趋势与混合线不同，表明大部分分馏过程在混合后进行。在岩浆上升过程中发生的混合片，而分馏是在岩浆房的侵位过程中产生的化学多样性的主导过程。S型花岗岩可能形成于伸展构造环境。
The Murrumbidgee Batholith is typical of other composite S-type granite batholiths in the Lachlan Fold Belt (LFB) of southeastern Australia, consisting of discrete peraluminous, cogenetic granite suites. However, it contains a unique suite (Murrumbucka) at its southern extremity that has chemical and petrological features transitional with metaluminous I-type granites. Previous detailed structural and metamorphic studies have shown that the batholith is tilted northward, exposing subvolcanic plutons in the north and root zones in the south, originally located at depths of ∼10 km. A transition from mafic, foliated, sheet-like granites in the south to felsic, generally less foliated, homogeneous granites in the north is consistent with magma ascent via subvertical, structurally controlled sheets to emplacement in an overlying magma chamber. In the inferred root zones, the Murrumbucka Suite hosts migmatitic metasedimentary and gabbroic rocks, both of which have transitional contacts and show evidence for mafic–felsic interaction with the Murrumbucka Suite. The migmatites extend southward to become part of the high-T, low-P Cooma Metamorphic Complex, which contains a core of heterogeneous remobilised diatexitic granite (Cooma Suite), lenses of which also occur throughout the southern (deeper) parts of the Murrumbidgee Batholith. The composition of the most mafic rocks from the batholith lies on a chemical tie-line between Cooma Suite granites and the gabbros, for almost all elements. This chemical coincidence is interpreted to reflect derivation of parental Murrumbidgee S-type granite magmas by bulk mixing between a felsic (crustal) and mafic (mantle-derived) component, consistent with Sr and Nd isotopic results and field observations. Based on the tie-lines and simple numerical modelling, the Murrumbucka suite is estimated to be a 50:50 mix of mantle- and crustal-derived magmas, whereas the more peraluminous and widespread Clear Range Suite, which is very typical of Lachlan S-type granites, is modelled as a 40:60 mix. Chemical variation trends diverge from the mixing lines and suggest that much of the fractionation process operated after mixing. Mixing occurred within sheets during the ascent of the magmas, whereas fractionation was the dominant process generating chemical diversity during emplacement in the magma chamber. Generation of the S-type granites probably occurred in an extensional tectonic environment.