SAPPHIRINE BEARING XENOLITHS OF KERGUELEN (INDIAN OCEAN) : EVIDENCE FOR CONTINENTAL NUCLEATION IN OCEANIC WITHIN-PLATE SETTING
M. Gregoire, C. Nicollet, J.Y. Cottin, N. Mattielli, D. Weis, N. Shimizu and A.Giret
Université Jean-Monnet, Laboratoire de Pétrologie, CNRS-UA N 10, 23 rue du Dr Paul Michelon, 42023 Saint-Etienne, France
Amongst UB-B xenoliths from the Kerguelen Archipelago, a group of wehrlites, lherzolites, websterites, metagabbros and anorthosites, characterized by the presence of Al-diopside and Al-spinel, is remarkable because of its relative abondance (45% of the sampled inclusions) and because of the occurence of sapphirine in some of basic xenoliths. These rocks present a predominant polygonal granoblastic microtexture and important development of coronites and symplectites. Some of them show a primitive magmatic layering. The sapphirine coexists with garnet in some xenoliths, but other samples are garnet free. The sapphirine bearing rocks are Mg rich (80.4 > MgO/MgO+FeOtot > 73 . 9). The presence of sapphirine and garnet result from a few equilibria reactions such as:
0.9 opx + 1. 1 plag. + 3.2 sp. <--> 1.0 sapph. + 1.3 cpx
in the garnet free rocks,
10.5 opx. + 7.5 sp. + 5.5 plag <--> 1.0 sapph. + 9.8 Gt,
5.7 opx. + 1.0 cpx + 2.6 sp. <--> 5.8 Gt,
9.9 -opx. + 3.3 sp. + 3.2 plag. <--> 1.0 cpx. + 7.6 Gt
in the garnet bearing rocks.
Sapphirine occurs in a few metamorphic rocks of granulite facies. Sapphirine bearing xenoliths have been described already in mantle xenoliths, but only in a continental environment. The xenoliths from the Kerguelen Archipelago provide the first occurence of sapphirine in rocks derived from the oceanic lithosphère.
The sapphirine bearing and others xenoliths from Al-diopside group have been metamorphosed in granulite facies conditions. They are related to early Mg-rich segreagtes of the Kerguelen tholeiitic-transitional melts reequilibrated in P-T conditions (0.9 to 1.2 GPa and 900-1000°C for the sapphirine beaing samples) of the lower part of the Kerguelen thickened oceanic crust and/or the upper mantle.
The recrystallization of cooling Mg-cumulates may be related to an underplating process or the subsidence of the archipelago. The second hypothesis seems hard to be concile with the presence of large volumes of differentiated plutonic and volcanic materials in the upper crust and probably important volumes of granulitic basic rocks in and below the lower crust which contribute to a generatly lower density of Kerguelen Islands.
Numerous recent models suggest that oceanic basalt plateaus may play an important role in the creation and development of continental crust. In accord with these models, we suggest that Kerguelen Archipelago and possibly the totality of.the Kerguelen plateau can constitute a continental nucleus in a pure oceanic environment.