Diverse Origins of Xenoliths from Seamounts at the Continental Margin, Offshore Central California

JOURNAL OF PETROLOGY VOLUME 48 NUMBER 5 PAGES 829^852 2007 doi:10.1093/petrology/egm003 Diverse Origins of Xenoliths from Seamounts at the Continental Margin, Offshore Central California A. S. DAVIS*, D. A. CLAGUE AND J. B. PADUAN MONTEREY BAY AQUARIUM RESEARCH INSTITUTE, 7700 SANDHOLDT ROAD, MOSS LANDING, CA 95039-9644, USA RECEIVED AUGUST 10, 2005; ACCEPTED JANUARY 31, 2007 ADVANCE ACCESS PUBLICATION MARCH 14, 2007 Xenoliths are samples of the mantle lithosphere underlying a volcano and/or the crust the host magma traversed. Their compositions provide information about the temperatures and pressures at which they originated or last equilibrated. They might record metasomatic processes that modify the lower lithosphere during rock^melt interactions. Mantle xenoliths have been described from various tectonic settings, including continental rifts (e.g. Frey & Prinz, 1978; Kempton, 1987; McGuire, 1988), island arcs (e.g. Takahashi, 1980), and ocean islands such as Hawaii (e.g. Sen & Presnall, 1986; Sen, 1988; Sen et al., 2005), the Canary Islands (e.g. Neumann, 1991; Neumann et al., 2000, and references therein), and the Society Islands (Qi et al., 1994). Some ocean island lavas containing mantle xenoliths also include fragments of old ocean crust (e.g. Clague & Chen, 1986; Fodor & Vandermeyden, 1988; Schmincke et al., 1998; Neumann et al., 2000). Other xenoliths are cumulates of ocean island (e.g. Sen & Presnall, 1986; Clague, 1987; Bohrson & Clague, 1988; Fodor & Moore, 1994; Fodor & Galar, 1997) or mid-ocean ridge magma chambers (e.g. Hekinian et al., 1985; Dixon et al., 1986; Davis & Clague, 1990). If the xenoliths ascend rapidly, there might be minimal interaction with their host magma. In contrast, if they are in prolonged contact with the melt, the xenoliths might be mineralogically and chemically modified. This study describes the petrography and mineral and host lava compositions of a diverse suite of xenoliths from Davidson and Pioneer seamounts, offshore central California. Unlike most intra-plate ocean island volcanoes, the seamounts are built on top of spreading center segments that were abandoned at the continental margin when the tectonic regime changed from subduction to a transform margin. The host lavas erupted millions of years after mid-ocean ridge basalt (MORB) volcanism ended (Davis et al., 1995, 2002). The xenoliths provide a window into the upper mantle and lower crust in this unusual environment. We use the mineral chemistry to identify and distinguish mantle and ocean crust cumulates *Corresponding author. Telephone: 831-775-1857. E-mail: ß The Author 2007. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: A diverse assemblage of small mafic and ultramafic xenoliths occurs in alkalic lava from Davidson and Pioneer seamounts located at the continental margin of central California. Based on mineral compositions and textures, they form three groups: (1) mantle xenoliths of lherzolite, pyroxenite, and dunite with olivine of 4Fo90; (2) ocean crust xenoliths of dunite with olivine 5Fo90, troctolite, pyroxenegabbro, and anorthosite with low-K2O plagioclase; (3) cumulates of seamount magmas of alkalic gabbro with primary amphibole and biotite and anorthosites with high-K2O plagioclase. The alkalic cumulates are genetically related to, but more evolved than, their host lavas and probably crystallized at the margins of magma reservoirs. Modeling and comparison with experimentally derived phases suggest an origin at moderate pressures (
05^09 GPa). The high volatile contents of the alkalic host lavas may have pressurized the magma chambers and helped to propel the xenolith-bearing lavas directly from deep storage at the base of the lithosphere to the eruption site on the ocean floor, entraining fragments of the upper mantle and ocean crust cumulates from the underlyingabandoned spreading center. KEY WORDS: basaltic magmatism; continental margin seamounts; geothermobarometry; mineral chemistry; xenoliths I N T RO D U C T I O N JOURNAL OF PETROLOGY VOLUME 48 from xenoliths related to the alkalic volcanism that built the seamounts. We estimate the depth of origin based on temperatures and pressures recorded by mineral equilibria in the xenoliths and draw inferences concerning magma generation and transport processes. GEOLOGIC A L S ET T I NG Davidson and Pioneer are two seamounts of a group of four located at the continental margin, offshore central California (Fig. 1). Morphologically similar seamounts are more abundant offshore southern and Baja California. Unlike typical ocean island volcanoes or near-ridge seamounts, all of the seamounts are complex NE^SWtrending ridges that reflect the ridge-parallel structure of the underlying oceanic crust (Davis et al., 2002). Davidson Seamount is built on a fossil spreading center based on symmetric magnetic anomalies of Chron 6, indicating an ocean crust age of about 20 Ma (Lonsdale, 1991). Mapped Chron 6C magnetic anomalies near Pioneer Seamount are not symmetrical about the seamount but suggest an age of about 24 Ma for the underlying ocean crust. Published 39 Ar/40Ar laser fusion ages indicate volcanism at about 12 Ma on Davidson and at 11Ma on Pioneer (Davis et al., 2002), younger by 8^13 Myr than the underlying oceanic crust. New Ar^Ar incremental heating results for some Davidson samples expand the age of volcanism at Davidson from
17 to
10 Ma (D. A. Clague, unpublished data), indicating that episodes of volcanism occurred on 3 to 10 Myr old ocean crust. Such prolonged volcanic activity to form the seamounts suggests very low magma supply rates and long hiatuses between eruptions, as suggested based solely on seamount morphology by Davis et al. (2002). Some whole-rock and glass chemistry data from Davidson and Pioneer seamounts were given by Davis et al. (2002), who presented petrography, Ar^Ar ages, and trace element and isotope compositions for lavas from the four seamounts offshore central California and for one located farther south. Volcanic rocks are predominantly alkalic basalt, hawaiite, and mugearite, but also include some tholeiitic basalt and rare trachyte. Radiogenic isotopes indicate a variably enriched MORB source (Davis et al., 2002; P. Castillo, personal communication). S A M P L I N G A N D A N A LY T I C A L M ET HODS The xenoliths occur in volcanic rocks that were collected by dredging on several cruises of the US Geological Survey (USGS) in 1976,1978, and 1979 and on dives of the remotely operated vehicle (ROV) Tiburon on three cruises of the Monterey Bay Aquarium Research Institute’s R.V. Western Flyer in 2000 and 2002. The xenoliths studied were selected to include the widest variety of minerals and textures, but they represent only a small fraction of the inclusions present NUMBER 5 MAY 2007 in the lavas. Whole-rock lava samples were analyzed by Xray fluorescence (XRF) at the GeoAnalytical Laboratory of Washington S (...truncated)