Cr and Sr: Keys to parental magmas and processes in the Bushveld Complex, South Africa

被引:36
作者
Cawthorn, R. Grant [1 ]
机构
[1] Univ Witwatersrand, Sch Geosci, ZA-2050 Wits, South Africa
基金
新加坡国家研究基金会;
关键词
Bushveld Complex; parental magmas; orthopyroxene compositions; plagioclase compositions; Cr; Sr;
D O I
10.1016/j.lithos.2006.09.004
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
Large layered intrusions are almost certainly periodically replenished during their protracted cooling and crystallization. The exact composition(s) of the replenishing magma(s) in the case of the Bushveld Complex, South Africa, has been debated, mainly on the basis of major element composition and likely crystallization sequences. The intrusion is dominated by orthopyroxene and plagioclase, and so their Cr and Sr contents, and likely partition coefficient values, can be used to re-investigate the appropriateness of the various proposed parental magmas. One magma type, with about 12%, MgO, 1000 ppm Cr and 180 ppm Sr, can explain the genesis of the entire Lower and Critical Zones. A number of other magma compositions proposed to produce the Critical Zone fail to match these trace-element constraints by being too poor in Cr. A fundamentally different magma type was added at the base of the Main Zone, but none of the proposed compositions is consistent with the trace-element requirements. Specifically, the Cr contents are higher than predicted from pyroxene compositions. A further geological constraint is demonstrated from a consideration of the Cr budget at this level. There is an abrupt decrease from about 0.4% to 0.1% Cr2O3 in orthopyroxene across this Critical Zone-Main Zone transition. No realistic proportions of mixing between the residual magma, at the top of the Critical Zone and any proposed added magma composition can have produced a composition that could have crystallized these low-Cr orthopyroxenes. Instead, it is suggested that the resident magma from the Upper Critical Zone was expelled from the chamber, possibly as sills into the country rocks, during influx of a dense, differentiated magma. Near the level of the Pyroxenite Marker in the Main Zone, there is further addition of a Ferrobasaltic magma, with 6% MgO, 111 ppm Cr and 350 ppm Sr, that is consistent with the geochemical requirements. (c) 2006 Elsevier B.V. All rights reserved.
引用
收藏
页码:381 / 398
页数:18
相关论文
共 67 条
[1]  
[Anonymous], 1994, S AFR J GEOL
[2]   THE DISTRIBUTION OF CHROMIUM AMONG ORTHO-PYROXENE, SPINEL AND SILICATE LIQUID AT ATMOSPHERIC-PRESSURE [J].
BARNES, SJ .
GEOCHIMICA ET COSMOCHIMICA ACTA, 1986, 50 (09) :1889-1909
[4]   A PROCEDURE FOR CALCULATING THE EQUILIBRIUM DISTRIBUTION OF TRACE-ELEMENTS AMONG THE MINERALS OF CUMULATE ROCKS, AND THE CONCENTRATION OF TRACE-ELEMENTS IN THE COEXISTING LIQUIDS [J].
BEDARD, JH .
CHEMICAL GEOLOGY, 1994, 118 (1-4) :143-153
[5]  
Blundy J, 2003, EARTH PLANET SC LETT, V210, P383, DOI [10.1016/S0012-821X(03)00129-8, 10.1016/S0012-821X(03)-00129-8]
[6]   Experimental study of a Kiglapait marginal rock and implications for trace element partitioning in layered intrusions [J].
Blundy, J .
CHEMICAL GEOLOGY, 1997, 141 (1-2) :73-92
[7]   CRYSTAL-CHEMICAL CONTROLS ON THE PARTITIONING OF SR AND BA BETWEEN PLAGIOCLASE FELDSPAR, SILICATE MELTS, AND HYDROTHERMAL SOLUTIONS [J].
BLUNDY, JD ;
WOOD, BJ .
GEOCHIMICA ET COSMOCHIMICA ACTA, 1991, 55 (01) :193-209
[8]   DENSITY CALCULATIONS FOR SILICATE LIQUIDS .1. REVISED METHOD FOR ALUMINOSILICATE COMPOSITIONS [J].
BOTTINGA, Y ;
WEILL, D ;
RICHET, P .
GEOCHIMICA ET COSMOCHIMICA ACTA, 1982, 46 (06) :909-919
[9]   LOWER ZONE OF EASTERN BUSHVELD COMPLEX IN OLIFANTS RIVER TROUGH [J].
CAMERON, EN .
JOURNAL OF PETROLOGY, 1978, 19 (03) :437-462
[10]   THE UPPER CRITICAL ZONE OF THE EASTERN BUSHVELD COMPLEX - PRECURSOR OF THE MERENSKY REEF [J].
CAMERON, EN .
ECONOMIC GEOLOGY, 1982, 77 (06) :1307-1327