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Magmatic Differentiation Processes in Saucer-Shaped Sills: Evidence from the Golden Valley Sill in the Karoo Basin, South Africa

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Magmatic Differentiation Processes in Saucer-Shaped Sills: Evidence from the Golden Valley Sill in the Karoo Basin, South Africa Magmatic differentiation processes in saucer-shaped sills: Evidence from the Golden Valley Sill in the Karoo Basin, South Africa Christophe Y. Galerne* PGP—Physics of Geological Processes, University of Oslo, P.O. Box 1048 Blindern, 0316 Oslo, Norway, and Steinman Institute, Department of Geophysics, University of Bonn, Nussallee 8, D-53115 Bonn, Germany Else-Ragnhild Neumann Ingrid Aarnes PGP—Physics of Geological Processes, University of Oslo, P.O. Box 1048 Blindern, 0316 Oslo, Norway Sverre Planke PGP—Physics of Geological Processes, University of Oslo, P.O. Box 1048 Blindern, 0316 Oslo, Norway, and VBPR—Volcanic Basin Petroleum Research, Forskningsparken, Oslo, Norway ABSTRACT 1. INTRODUCTION crystallization. The most recurrent geochemical profi les observed in such sills are I-, D-, and Analysis of compositional variations Bulk composition profi les of basic-ultrabasic S-shaped (Fig. 1; e.g., Gibb and Henderson, along profi les from tholeiitic sills provides sills have been investigated for a long time in 1992; Latypov, 2003a). The nomenclature is insights into syn- and post-emplacement order to understand the magmatic differentiation based on the variations in the whole-rock Mg# magmatic differentiation processes. We processes associated with their emplacement and ( = cation proportion 100 × Mg/[Mg + Fetotal]) present here 18 whole-rock compositional profi les sampled from a saucer-shaped sill emplaced in the Karoo Basin (South Africa), I-shaped C-shaped D-shaped S-shaped the Golden Valley Sill. We show that dif- ferent compositional profi le patterns pre- Top Top Top Top viously described in basic-ultrabasic sills may be found in different parts of a single saucer-shaped sill. The detailed examina- tion of the mineral grain assemblage and compositions suggests that processes taking place in hundred-meter-thick sills relate to Sill thickness Sill thickness Sill thickness Sill thickness early and late fractional crystallization. Our observations in the Golden Valley Sill sug- gest that a signifi cant part of fractionation Base Base Base Base takes place at a late stage of cooling when a Mg# Mg# Mg# Mg# crystalline skeleton or mush zone is formed. We show that porous fl ow of interstitial melt Uniform Positive segregation Negative segregation driven by forces related to the particular profile shape geometry (saucer-shaped) of the sill may Non-uniform profile shapes result in a post-emplacement compositional evolution. We propose that the process of Figure 1. Characteristic shapes of compositional profi les defi ned on the basis of Mg# (dif- post-emplacement melt fl ow regionally ferentiation index, Mg# = molar 100 ×Mg/[Mg + Fetotal]) variations along vertical sections overprinted compositional patterns pro- across igneous intrusives. The I-shaped profi le is a uniform profi le showing no chemical duced by earlier crystal segregation from variation across the sill thickness. Nonuniform profi le patterns in relatively thin sheet intru- the cooling magma at fl uid-like stages dur- sions (100 m thick, e.g., Golden Valley Sill Complex) are characterized by D- and S-shaped ing the emplacement. compositional profi les also qualifi ed as reversed or inverse segregation. C-shaped composi- tional profi les are characteristics of large magmatic plutons, also qualifi ed as positive (i.e., expected) segregation. *[email protected]. Geosphere; June 2010; v. 6; no. 3; p. 163–188; doi: 10.1130/GES00500.1; 18 fi gures; 2 tables; 3 supplemental tables. For permission to copy, contact [email protected] 163 © 2010 Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/6/3/163/3338493/163.pdf by guest on 02 October 2021 Galerne et al. from fl oor to roof of the sills. The term I-shaped along the profi les. The profi les are classifi ed on 182.7 ± 0.3 Ma of a sill in the GVSC (Svensen profi le is used to describe sills showing little the basis of the established nomenclature, i.e., et al., 2007) implies that these sills are contem- fi eld, petrographic, and geochemical evidence I-, D-, and S-shaped profi les. Profi les that did poraneous with the Lesotho Flood basalts which of differentiation throughout the height of the not fi t into the established nomenclature were are dated to ca. 183 Ma (Duncan et al., 1997). sill (Mangan and Marsh, 1992; Marsh, 1996). termed X-shaped profi les. Mineral chemical Sill complexes in the Karoo Basin represent a The name D-shaped profi le was introduced to variations are illustrated for one profi le. The last signifi cant portion of the Jurassic Karoo igne- characterize sills with the least-differentiated section gives a review of the main results and ous province (Galerne et al., 2008; Marsh and composition at the sill center (i.e., highest discusses the various types of compositional Mndaweni, 1998). Saucer-shape sills are com- Mg#) and most evolved composition at the profi les found in the Golden Valley Sill in terms mon structures in sill complexes (Hansen and sill margins (i.e., lowest Mg#). Finally, an of cooling and crystallization processes. Cartwright, 2006; Hansen et al., 2004; Hansen S-shaped profi le shows S-shaped variations in et al., 2008; Polteau et al., 2008b; Thomson and Mg# upwards through the sill (Frenkel et al., 2. GEOLOGY AND GEOCHEMICAL Hutton, 2004). A saucer-shaped sill is character- 1988, 1989; Fujii, 1974; Gray and Crain, 1969; BACKGROUND OF THE GOLDEN ized by a fl at inner sill connected outward and Marsh, 1989). VALLEY SILL COMPLEX upward to a ring of discordant inclined sheets, D- and S-shaped profi les observed in rela- which are often terminated in a fl at outer sill. tively thin sills (100 m) pose a problem because The GVSC consists of a group of sills and The GVSC has intruded the Beaufort Group they are “inversed” from what is expected of dykes emplaced in an undeformed sequence of in the Karoo Basin sedimentary sequences. cooling magma. Large layered mafi c intru- sandstone and shale (Fig. 2). A concordia age of Like numerous sills in the Beaufort Group, the sions have typically mafi c margins and felsic cores (normal zoning) forming C-shaped geo- chemical profi les (Fig. 1; Skaergaard intrusion, 26°10 E 26°20 E 26°30 E 26°40 E Naslund, 1984). C-shaped zoning or composi- A' A tional profi le is interpreted as the result of in situ processes of fractional crystallization (Rice, Glen -31°80 N 1981). The lack of C-shaped profi les in rela- Sill tively thin sills is signifi cant and suggests that MV Sill (GS) processes occurring in a large magmatic body (MVS) are different from those in thin sills (100 m). An additional problem is that available data are Harmony generally restricted to one to three profi les in Sill a sill (e.g., Richardson, 1979). Consequently, Golden Valley Dyke (GVD)Golden (HS) we usually do not know if a single sill contains A -31°90 N different types of compositional profi les and Valley if there are variations in type of profi le along Sill the boundary of sills. Such information is, (GVS) N profiles however, of fi rst-order importance in order to P17 understand the magmatic differentiation pro- Morning cesses that occur in sills. P18 Sun (MS) Our strategy in this study was to document the vertical and lateral compositional and tex- Km -32°00 N tural variations within a single well-exposed 0 saucer-shaped sill in order to reveal the mag- 4 matic differentiation processes within this sill. B 26°S Our study object was the Golden Valley Sill o o 30026 Km 10 E SOUTH 26 20 E 26°30 E 26°40 E in the Golden Valley Sill Complex (GVSC), AFRICA Karoo Basin, South Africa (Aarnes et al., 2008; A Morning Glen Sill A' KAROO Galerne et al., 2008; Polteau et al., 2008a). The 30°S Sun (MS) Golden Valley (GS) BASIN excellent exposure of the Golden Valley Sill Sill (GVS) allowed systematic sampling along vertical pro- Harmony GVSC fi les all along the inclined sheets/outer sill. We Morning ? Sill (HS) 34°S GVD ? ? ? report here whole-rock analyses on 18 profi les 18°E22°E 26°E 27°E Sun (MS) (Fig. 2A; fi ve samples per profi le in average) together with mineral chemistry on representa- Figure 2. Summary of the geochemical architecture of the Golden Valley Sill Complex tive profi les. (GVSC) after Galerne et al. (2008). (A) Representation of the four different magma batches This paper has three main sections: The fi rst identifi ed in the GVSC. These are represented by different colors and are stratigraphically section presents the geology and geochemis- positioned in cross section A–A′ (Galerne et al., 2008). Profi le locations are indicated by try of the Golden Valley Sill followed by sam- brown and green cylinders; the colors indicate the geochemical signature of the Golden Val- pling strategy and methodology. The next sec- ley Sill and the Morning Sun sill, respectively. (B) Location of the GVSC in the Karoo Basin tion gives a systematic overview of signifi cant among the dolerite intrusives (represented in black). The location of the Lesotho Flood textural, petrographic, and chemical variations basalt remnant is indicated by a gray color. 164 Geosphere, June 2010 Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/6/3/163/3338493/163.pdf by guest on 02 October 2021 Magmatic differentiation processes in saucer-shaped sills GVSC’s main sill morphology is saucer-shaped. al., 2008). The 100-m-thick Golden Valley Sill istic of cooling features in the top contact of Four major saucer-shaped sills, all elliptical in is the best exposed sill in the GVSC.
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