THE EFFECTS OF PROGRESSIVE PARTIAL MELTING OF METASEDIMENTARY ROCKS ON STRAIN PARTITIONING: A CASE STUDY FROM THE SOUTHERN SIERRA NEVADA BATHOLITH, CALIFORNIA

The upper metapelite unit of the King Sequence, in response to the emplacement of the Goat Ranch granodiorite intrusion, southern Sierra Nevada, CA, had undergone partial melting. Partial melting and in situ segregation of leucogranitic melt led to the formation of migmatites within the Jurassic metapelite that is in direct contact with the Goat Ranch granodiorite. Field observations show that migmatites have various structure styles due to progressive partial melting of a pelitic source. Therefore, it may yield important insights into how partial melting help to develop strength contrast and affect strain partitioning in metasedimentray rocks.

Close to the contact, leucosomes form interconnecting network, while solid residues occur as segregated and deformed lentoids. With the distance away from the contact, leucosomes become more and more isolated and finally as isolated pockets in the metapelite. This transition in structural styles of the migmatite suggest that loading framework for the migmatite changed from LBF structure to IWL structure with increasing extent of partial melting. Strain studies of the adjacent massive quartzite and marble units suggest an extremely high stretching strain up to 1000% as a result of diapiric emplacement of the Goat Ranch granodiorite intrusion. In contrast, the strain recorded by solid residues within the migmatite only suggests 200% to 300% stretching strain, which is similar to the strain recorded by the deformed pebble conglomerates interbedded by clay-rich layers. The differences or inconsistencies may be due to the presence of melt within the metapelite during deformation. Leucogranitic melts represented by leucosomes behaved essentially similar to the weak clay-rich layers in the pebble conglomerate. The presence of melt is likely to (1) help to form the strength contrast between melts and solid residues, and (2) affect the strain partitioning such that melt will accommodate more strain due to its low strength than the solid residue, given the same stress.