XENOLITH FRAGMENTATION, SCREEN FORMATION & ASSIMILATION IN THE TILTED WOOLEY CREEK BATHOLITH: A DOG'S BREAKFAST OF PROCESSES DURING MAGMA EMPLACEMENT

The formation and evolution of xenoliths and screens in plutonic rocks remains controversial. We present new field observations and petrologic/geochemical data that suggest that xenolith incorporation, screen formation, and assimilation are widely distributed processes in the 158-155 Ma Wooley Creek batholith (WCb), a tilted intrusion in the Marble Mountains Wilderness, northern CA.

Outcrop mapping along the Salmon River demonstrates that the southern portion of the WCb (shallow, “roof” portions of the magma chamber; p=~300 MPa) formed by pulses of sheet-like bodies that penetrated calc-silicate and chert-argillite host rocks. Individual sheets consist of gabbro, quartz diorite, tonalite and granodiorite and contain numerous screens and xenoliths of the host rocks ranging in size from mm to 100’s of m in area. Some xenoliths have internal structures that are discordant to the orientations of similar structures in the host rocks; others appear to maintain the host rock stratigraphy and internal structural/bedding orientations and are termed screens. In both cases, pelitic layers within schistose layers are migmatitic and contain injection and in-situ veins of qt + kf (?) + pl ± amph ± px. Amphibole gabbro host rocks appear to increase in modal px abundance within 2-10 meters of the migmatitic screens/xenoliths. Regional and outcrop mapping in the central portions of the WCb indicate that xenoliths/screens of similar aerial extents are preserved along a trend that separates the deeper (northern; p=550 to > 750 MPa) exposures of the pluton from shallower (southern) exposures.

We draw the following testable conclusions from these observations: 1) xenoliths and screens within the WCb melted in-situ during emplacement of parent magmas within the evolving magma chamber, even in shallow crustal environments near the roof; 2) xenoliths were able to freely rotate in the magma after incorporation; 3) apparent pyroxene abundance in amphibole gabbro near calc-silicate and chert argillite screens/xenoliths may be due to assimilation of partial melts derived from pelitic layers within the calcareous xenoliths/screens; and 4) screens/xenoliths preserved in the central portions of the WCb may represent ghost stratigraphy of the host rocks and record changing mechanisms and locations of magma emplacement through time.