The Jurassic gabbroic intrusions of Utpostane and Muren: insights into Karoo-related plutonism in Dronning Maud Land, Antarctica

Abstract

Middle Jurassic continental flood basalts of Vestfjella, western Dronning Maud Land are cut by gabbroic intrusions that represent rare exposures of Karoo-related mafic plutons in Antarctica. The gabbros and numerous associated dolerites indicate high magmatic activity along the continental margin of Dronning Maud Land during the break-up of Gondwana. The scattered nunataks of Utpostane (~25 km 2 ) are dominated by olivine gabbronorite and olivine mela-gabbronorite, which can be grouped into four zones. The Utpostane intrusion exhibits inclined sheet-like geometry, is at least ~3 km thick and shows moderate enrichment of incompatible element contents from its base towards the exposed roof contact (e.g. Zr from 15 to ~60 ppm). High MgO contents (~8-36 wt%) indicate that the parental magma of Utpostane was more primitive than a typical Karoo tholeiite (MgO ~6 wt%). At Muren, gabbroic outcrops record a cross section of a ~1.3 km thick inclined sheet-like intrusion. The intrusion can be divided to two main units, the upper and lower zones, which are dominated by olivine gabbro and gabbronorite, respectively. Parental melts of the geochemically differentiated upper zone and the homogeneous lower zone of Muren were typical lowMgO Karoo tholeiites, but they were chemically distinct and were emplaced as separate magma pulses. With these data we address the petrogenesis of the gabbroic rocks and discuss their regional geological significance. Geological backgroundVestfjella is located at the continental margin of western Dronning Maud Land. The exposed bedrock of Vestfjella is composed entirely of Phanerozoic rocks, mainly Jurassic continental flood basalts (Hjelle & Winsnes 1972). Elsewhere in Dronning Maud Land, Precambrian basement dominates and Jurassic basalts only occur at the southern Kirwanveggen and northern Heimefrontfjella at the East Antarctic Plateau escarpment (Fig. 1). The basalts are underlain by up to ~160 m thick Palaeozoic sedimentary sequence (Juckes 1972). The basement consists of an Archaean (Grunehogna) craton (Halpern 1970, Barton et al. 1987 and Mesoproterozoic (Maud) metamorphic belt (Groenewald et al. 1995). The Archean-Proterozoic lithospheric boundary coincides with the Jutulstraumen graben and, based on aeromagnetic measurements, Corner (1994) has interpreted the boundary to curve eastward from Pencksökket and run close to northern Vestfjella (Fig. 1).The Kirwanveggen lavas at the escarpment are geochemically homogeneous low-Ti tholeiites (Harris et al. 1990). In contrast, the compositionally variable lavas of Vestfjella constitute four different magma series (Luttinen & Furnes 2000). The base of the ~1 km thick sub-horizontal Vestfjella lava sequence is unexposed, but basalt-hosted xenoliths suggest subsurface occurrence of Archaean basement as well as Permian sedimentary rocks (Luttinen & Furnes 2000). Minor sedimentary intercalations occur within the lava succession.The presence of gabbroic plutons at Utpostane and Muren ( Fig. 1) and the abundance of dolerite dykes and sills ind...

Enriched continental flood basalts from depleted mantle melts: modeling the lithospheric contamination of Karoo lavas from Antarctica

“…The intrusive rocks of Vestfjella include gabbros (Vuori and Luttinen 2003), ferropicrite dikes Luttinen 2008, 2010;, and lamproite dikes (Luttinen et al 2002). Most of the intrusive rocks, however, are tholeiitic dikes that exhibit large geochemical variations, from incompatible element-enriched CT1-type (ε Nd(t) = −18) to dikes relatively depleted in the most incompatible elements (the so-called "Low-Nb" magma type; ε Nd(t) from +7 to +8; Fig.…”

Origin, Distribution and Evolution of Plume Magmatism in East Antarctica

“…The Mesozoic metabasites of the Schirmacher Oasis (Fig. 2) are identical in composition to the dolerites of the dike complex of the Muren massif in western DML (Vuori & Luttinen, 2003) and slightly different from the ancient anorthosite dikes occurring at the Schirmacher Oasis and often showing similar strikes (Belyatsky et al, 2002). In general, the crystallization sequence of the basalts is magnesian olivineplagioclase-clinopyroxene.…”

“…eNd and ( 87 Sr/ 86 Sr)t -initial isotope composition corresponding sample at the time of dike emplacement (170 Ma). (Table 1), (2) dikes of the Muren region of DML after (Vuori & Luttinen, 2003), and (3) ancient anorthosite dikes from the Schirmacher Oasis region (Belyatsky et al, 2002). differentiated Jurassic lavas of the western DML .…”

“…The westernmost occurrences of this magmatism are basalts and dolerites from the western part of the Dronning Maud Land (DML), which have tholeiitic compositions and are geochemically enriched to a varying degree (Harris et al, 1990;Luttinen & Furnes, 2000;Vuori & Luttinen 2003). The magmatic complexes of DML (Vestfjella, Heimefrontfjella, and Kirwanveggen mountains and the Ahlmannryggen Plateau) were formed within a narrow time interval between 183 and 175 Ma, with the maximum magmatic activity at ca.…”

Apatite single‐grain (U‐Th)/He data from Heimefrontfjella, East Antarctica: Indications for differential exhumation related to glacial loading?

“…The base of the Vestfjella lavas is unexposed. Basalt hosted sedimentary fragments and minor sedimentary intercalations within the lava successions suggest the presence of Permian sedimentary cover beneath the volcanic rocks [ Luttinen and Furnes , 2000; Vuori and Luttinen , 2003]. A lava flow and a doleritic sill from the Schivestolen yielded 40 Ar/ 39 Ar whole rock ages of 172.4 ± 2.1 and 182.1 ± 1.9 Ma [ Brewer et al , 1996].…”