TWO GABBROIC DIKES FROM THE KAROO LIP EXHIBIT INCREASING εND TOWARDS THE CONTACT WITH THE WALLROCK AND RECORD TWO DISTINCT STORIES OF DIFFERENTIATION

Two gabbroic dikes with thicknesses of 350 m (East-Muren, EM) and 520 m (West-Muren, WM) are found crosscutting continental flood basalts (CFBs) related to the ~180 Ma Karoo large igneous province (LIP) in Vestfjella, western Dronning Maud Land, Antarctica. The dikes are geochemically and temporally related to Karoo CFBs, but represent distinct magma types in comparison to the lava flows they are crosscutting. Both dikes exhibit unusual geochemical characteristics: initial ε_Nd values increase towards the contact with the basaltic wallrock (in EM from -15.3 to -11.8 and in WM from -7.2 to -5.5). In the case of EM, incompatible trace elements correlate negatively and compatible trace elements (and MgO) positively with ε_Nd; in the case of WM, these correlations are opposite.

We suggest two distinct scenarios for the petrogenesis of the dikes. Both parental magmas experienced assimilation of Precambrian basement deeper in the crust before intruding into the CFBs, which lowered the ε_Nd of the magmas to highly negative values. The most primitive and least contaminated magmas of EM left their imprints in the border zones of the intrusion and the central parts were later intruded by magmas from the same chamber that then had experienced further assimilation of Precambrian crust. In the case of WM, the whole magma body that had assimilated Precambrian crustal materials intruded into the basalts almost simultaneously. The thermal energy available in the relatively large magma body enabled partial melting and assimilation of the hydrothermally altered basaltic wall rock with higher ε_Nd (ca. +2) near the contact zones.

We test and give constraints on the presented scenarios with the help of energy-constrained assimilation-fractional crystallization modeling using the Magma Chamber Simulator. Our study indicates that assimilation of early and relatively depleted LIP crust may cause unconventional geochemical changes in later magma pulses.