Geologica Acta, Vol.10, Nº 1, March 2012, 33-41 DOI: 10.1344/105.000001706 Available online at www.geologica-acta.com The Stillwater Complex chromitites: The response of chromite crystal chemistry to magma injection 1 2 2 3 1 D. LENAZ G. GARUTI F. ZACCARINI R.W. COOPER F. PRINCIVALLE 1 Department of Mathematics and Geosciences Via Weiss 8, 34127 Trieste, Italy. Lenaz E-mail: [email protected] 2 Department of Applied Geosciences and Geophysics Peter Tunner Strasse 5, 8700 Leoben, Austria 3 Department of Geology Lamar University, 77710 Beaumont TX, USA ABS TRACT Nineteen chromite crystals from the A, B, E, G, H, J and K chromitite layers of the Peridotite Zone of the Stillwater Complex (Montana, USA) have been studied by means of X-ray single crystal diffraction and microprobe analyses. The results show that samples from the basal A layer are quite different from the others showing very high oxygen positional parameter u (0.2633-0.2635) and Ti- contents (0.059-0.067apfu). Mg# values are within the range 0.21-0.23 while for the other chromites it is in the range 0.45-0.47. Moreover, for the other samples, according to the structural parameters, two groups have been identified. The first one comprises samples of layers B, E and G, the second includes H, J and K layer samples. It is supposed that high Fe2+ and Ti contents of A layer samples are due to the post-crystallization reaction with interstitial liquid. This fact allowed a very slow cooling rate as evidenced by the high u values. The fractionation of evolved magma from within the intrusion and pulse of a new magma bringing more chromium into the chamber lead to Cr- and Fe3+ -rich compositions and consequently to the increase of the cell edges. The decrease of u values seems to be related to the Cr+Fe3+ and/or Al contents. KEYWORDS X-ray single crystal diffraction. Structural refinement. Cr-spinel. Stillwater. INtrODUctiON of chromite, in terms of chemical exchanges among Cr, Al, Mg, Fe2+, Fe3+ and Ti, has become a common Cr-bearing spinels are widely regarded as important discriminating parameter to distinguish among podiform petrogenetic indicators. Several petrological studies have chromitites associated with residual mantle in sub-oceanic shown general relations between spinel chemistry, rock and sub-continental settings, stratiform chromitites in type and processes, and new understanding has been cratonic layered intrusions and chromite segregations achieved of igneous/metamorphic events from analyses of in Ural-Alaskan type complexes (Irvine, 1967; Thayer, spinel-bearing assemblages. In particular, the composition 1970; Sack, 1982; Dick and Bullen, 1984; Sack and 33 D . L E N A Z e t a l . Crystal chemistry of Stillwater chromites Ghiorso, 1991; Roeder, 1994; Stowe, 1994; Barnes and GEOLOgical settiNG Roeder, 2001). Recently, the relations between chemistry and structural parameters in natural spinels have been The Stillwater Complex is located in southern Montana considered by several authors, providing a new approach (USA) and consists of a more than 6km thick sequence of to the understanding of spinel crystallization mechanisms ultramafic to mafic layered rocks along the northern front in magmatic and metamorphic environments (Della of the Beartooth Mountains. A Sm-Nd isochron on mineral Giusta et al., 1986; Princivalle et al., 1989; Carraro, separates from a gabbronorite near the West Fork Adit 2003; Bosi et al., 2004; Uchida et al., 2005; Lenaz et al., portal gave a crystallization age of 2701±8Ma (DePaolo 2007, 2009, 2010). Structural data, essentially cell edge, and Wasserburg, 1979). Nunes (1981) determined an age of a0, and oxygen positional parameter, u, allow calculation 2713±3Ma on zircons from the Basal series and Premo et of the cation distribution between the tetrahedral and al. (1990) determined a U-Pb zircon age of 2705±4Ma for octahedral sites in the spinel structure. The cell edge a0 the dike/sill suite that is associated with the Basal Series, is apparently influenced by the spinel mineral chemistry, indicating that the sills are coeval with the main Complex mainly the Al and Cr content. In contrast, the spinel emplaced at a depth of about 10-15km (McCallum, 1996). oxygen coordinate u seems to be the most discriminating parameter, depending mainly on distribution of Mg and The Complex is divided into three major stratigraphic Al between tetrahedral and octahedral sites (Princivalle series (Fig. 1) based on the mineralogy of the cyclic units: et al., 1989). Thus the u parameter is influenced by the the Basal Series, the Ultramafic Series and the Banded degree of ordering of the spinel structure, becoming a Series. The Basal Series (average ~160m) separates the powerful indicator of physical parameters prevailing in magmatic cumulates from footwall hornfels. It shows the chromite crystallizing system (Princivalle et al., 1989). great variability in terms of lithologic sequence, however, More recent studies, anyway, argued that u parameter predominantly consists of norite with minor peridotite, too can be related to the general chemistry (Lenaz et anorthosite and hornfels xenoliths, grading upwards into al., 2010). Studies have been previously performed on the basal bronzitite (McCallum, 2002). The appearance of crystal chemistry of Cr-spinels from mantle nodules, sub- extensive cumulus olivine marks the base of the Ultramafic continental mantle massifs, ophiolites, kimberlites (Della Series that extends about 2000m in thickness (Mountain Giusta et al., 1986; Princivalle et al., 1989; Carraro, 2003; View section) up to the first appearance of plagioclase Bosi et al., 2004; Uchida et al., 2005; Lenaz et al., 2007, as a cumulus phase (Raedeke and McCallum, 1984). 2009, 2010) and detrital in sedimentary environments The Ultramafic Series is divided into a lower, Peridotite (Carbonin et al., 1999; Lenaz and Princivalle, 1996, 2005; Zone, and upper, Bronzitite Zone. The Peridotite Zone, in Lenaz et al., 2002) its turn, is divided into six Megacyclic groups (numbered from the base as I to VI) comprising lithologically similar Chemical structural studies on spinels from stratiform Multicyclic units of olivine–chromite–orthopyroxene chromitites are limited to the case of the Bushveld layered cumulus layers, which are separated from each other by Complex (Lenaz et al., 2007). The most relevant conclusion, magmatic unconformities (Cooper, 1997, 2002). Towards in that case, was that small differences in oxygen positional the upper part of the Peridotite Zone, bronzite becomes the parameter, u, were apparently related to variations in the most abundant cumulus phase marking the transition to the cooling rate estimated from the filling and crystallization Bronzitite Zone. The overlaying Banded Series (~4000m) model of the Bushveld magma (Cawthorn and Walraven, comprises all rocks in which cumulus plagioclase is a 1998). Moreover, it was noticed that distinctive structural major constituent, consisting of norite, gabbronorite, parameters were recorded for Cr-spinels having peculiar gabbro, troctolite and anorthosite (McCallum, 2002). thermal histories such as those from the Merensky Reef and the Tweefontein pipe of the Bushveld Complex (Lenaz Successive massive chromite layers are restricted to et al., 2007). the Peridotite zone being traditionally referred to as A (lowermost) through K (uppermost), of which layers G This study reports the results of a structural-chemical and H have been mined in the past (Campbell and Murck, investigation of nineteen chromite crystals from seven 1993). They occur within olivine cumulates or olivine- chromitite layers located in the peridotite zone of the bronzite cumulates, generally located close to the base of Stillwater Complex (Montana, USA). Major purposes the Megacyclic groups. In the lowermost Megacyclic group of this work are: first, to increase the data base for Cr- I, a thick horizon enriched in hornfels inclusions has been spinels in chromitites from large, continental layered observed in drill cores separating the chromitite layer A intrusions; second, to verify what relationships do exist (Multicyclic unit A) from chromitites in the B Multicyclic between spinel structural parameters and chemical unit (Cooper, 1997, 2002). Other chromitite layers occur in behaviour and petrogenetic models proposed for the the Megacyclic groups IV (chromitite E), V (chromitites G, Stillwater Complex. H, I), and VI (chromitites J and K). Geologica Acta, 10(1), 33-41 (2012) 34 DOI: 10.1344/105.000001706 D . L E N A Z e t a l . Crystal chemistry of Stillwater chromites GENetic MODels FOR THE StillWater COMPleX orthopyroxenes decrease upsection throughout the Peridotite Zone of the Ultramafic Series (Lambert and Mineralogical and isotopic variations through the Simmons, 1987). sequence indicate that several magma types and multiple magma injections were responsible for the cumulate rocks Recently, Horan et al., (2001) analyzed Re-Os isotopes present in the Stillwater Complex (Raedeke and McCallum, of chromite separates within the Peridotite Zone. The 1984; Lambert and Simmons, 1987; Lambert et al., 1989; chromites show variable initial Os isotopic compositions Loferski et al., 1990; Campbell and Murck, 1993; Lipin, (γOs of +2.0 to +16.4) over the vertical extent of the 1993; McCallum, 1996, 2002; Horan et al., 2001; Spandler Peridotite Zone, implicating at least two sources of Os. et al., 2005). They argued the variations in γOs were caused by mixing of variable proportions of two magmas having different Os Raedeke and McCallum (1984)