ANALYZING THE EFFECTS OF PLUTON SIZE AND COMPOSITION ON SCHLIEREN-BOUND MAGMATIC STRUCTURES IN THE SIERRA NEVADA, CALIFORNIA

A variety of magmatic structures are recognized in plutons (e.g., schlieren-bound structures, enclaves, fabrics and faults), providing a record of magmatic processes during cooling and crystallization. Several hypotheses for the formation of schlieren-bound magmatic structures have been proposed, including formation by local dynamic magma flow. Previous studies have identified schlieren-bound structures in voluminous super-plutons (>650km²) with long-lived, complex thermal histories. This study explores the effects of cooling rate on schlieren formation by mapping plutons of different areal extents. Field data and petrography were used to record observations of schlieren orientation, cross-cutting relationships, and mineralogy between the Wright’s Lake Granodiorite, Kwl (~750km²), and the Pyramid Peak Granite, Jpp (~45km²) in the Twin Lakes region of the Desolation Wilderness.

Across a 3km transect, the smaller Jpp contained a greater abundance of schlieren (n=36 schlieren) compared to the Kwl (n=9 schlieren), indicating that the size (cooling rate) is not the only factor determining schlieren abundance. Additionally, schlieren are clustered locally across the transect at ~40-100 m scales in the Jpp. Stereonets of schlieren orientation show a NW-SE trend throughout the Jpp (strike/dip: 305/43) and the Kwl (strike/dip: 338/57), parallel to the contact and regional tectonic structures. Petrographic analysis revealed distinct compositions of the Jpp host and schlieren, with 5% biotite and opaque minerals in the host, compared to 60% in the schlieren sample. Hornblende in the Jpp host and schlieren is non-existent while a high content of hornblende (~30%) exists in the Kwl host and schlieren. Together with field observations, this illustrates the significance of host composition in determining schlieren composition. Mafic enclaves contain the same mineralogy whether they reside in the Jpp or the Kwl, supporting the idea that they are formed by a distinct magma. Pluton compositions and local magmatic conditions at the 10’s m-scale may significantly influence schlieren formation and abundance: more work is needed to investigate relationships to final pluton sizes. This study represents one approach towards understanding why schlieren are found in plutons worldwide but vary in type and density.