Assimilation and crystal accumulation in a mid-crustal magma chamber: the Sausfjellet pluton, north-central Norway

被引:30
|
作者
Barnes, CG
Dumond, G
Yoshinobu, AS
Prestvik, T
机构
[1] Texas Tech Univ, Dept Geosci, Lubbock, TX 79409 USA
[2] Norwegian Univ Sci & Technol, Dept Geol & Mineral Resources Engn, N-7491 Trondheim, Norway
基金
美国国家科学基金会;
关键词
diorite; assimilation; stoping; Caledonian; Norway;
D O I
10.1016/j.lithos.2004.04.036
中图分类号
P3 [地球物理学]; P59 [地球化学];
学科分类号
0708 ; 070902 ;
摘要
The Sausfjellet pluton is made up of two intrusive units emplaced into high-grade metamorphic rocks of the Helgeland Nappe Complex of the Uppermost Allochthon in the Norwegian Caledonides. The eastern part of the pluton intruded marble and less voluminous calc-silicate and pelitic rocks. The western half is hosted predominantly by semi-pelitic migmatite with intercalated marble. Remelting of the migmatite during pluton emplacement occurred in a thermal aureole as much as 1000 m wide. The early gabbroic unit forms the southeastern part of the body; it consists of homblende-bearing to homblende-rich gabbro and diorite which is thought to have crystallized from an H2O-rich andesitic parental magma. The younger dioritic unit underlies the central and western parts of the pluton, as well as a zone as much as 200 m wide that separates the rest of the pluton from its host rocks (herein the "annular zone"). The interior or central zone of the dioritic unit is pyroxene diorite that is locally interlayered with anorthosite. The western and annular zones are, by comparison, mineralogically heterogeneous. They range from diorite to quartz monzonite and from biotite-bearing two- and three-pyroxene assemblages to biotite-hornblende assemblages. Neither rock type nor mafic assemblage is correlated with position in the pluton or proximity to a contact. Stoped blocks of a distinctive coarse-grained diorite, as well as pyroxene-rich cale-silicates, are present in the gabbroic unit and the central zone of the dioritic unit. The few stoped blocks observed in the western zone of the dioritic unit are predominantly quartz-rich gneiss. Chemical variation in the central zone of the dioritic unit is interpreted to result from accumulation of pyroxenes+plagioclase from an H2O-Poor andesitic parent. These rocks have approximately constant 6 delta(18)O of + 6.6 +/- 0.2parts per thousand and lack evidence of in situ assimilation. Heterogeneities in the western and annular zones of the dioritic unit are reflected in variable, anomalously enriched incompatible element contents and in delta(18)O, which ranges from +6.7parts per thousand to +8.6parts per thousand. Petrologic models indicate that the magma parental to the central zone could also be parental to the western and annular zones. If so, evolution of the western and annular zone magma was by crystal accumulation and assimilation of metapelitic host rocks. As much as 20% of the mass of the western and annular zones can be ascribed to assimilated material, which apparently entered the magma by stoping. Therefore, the asymmetrical zoning of the pluton is due to differences in host rock compositions and the relative ability of the magma to assimilate its host rocks. (C) 2004 Elsevier B.V. All rights reserved.
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页码:389 / 412
页数:24
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