EPISODIC DOWNWARD CRUSTAL FLOW DURING TRIASSIC TO CRETACEOUS MAGMA SURGES IN THE CENTRAL SIERRA ARC

The Cinko, Jackass, Soldier, and Saddlebag lakes areas of the central Sierra region of California have significant downward displacement of Mesozoic volcanic rocks during episodes of Triassic, Jurassic, and Cretaceous pluton intrusion. Plutons and intruded metavolcanics share down-dip foliations and lineations. Host rocks record rotations to steep dips and strains up to 75% shortening and >100% extension.

In the Cinko Lake area, the Harriet Lake (102 Ma) and Fremont Lake (< 95 Ma) granodiorite plutons are intruded into slightly older metavolcanics (102-107 Ma; all radioisotopic ages quoted here are U-Pb zircon). Hornblende-plagioclase rims yield estimates of 684-726°C at 2.6 kb for the Fremont intrusion and 684-698°C at 2.5 kb for the Harriet Lake, corresponding to emplacement depths of 9-10 km.

The ~97 Ma Jackass Lakes pluton intrudes metavolcanics with ages ranging 98-103 Ma. Hornblende-plagioclase thermobarometry indicate emplacement at 3.0 kb (11 km) at 681°C.

In the Soldier Lake region, the 86 Ma Cathedral Peak, the 97 Ma Soldier Lake, and the 165 Ma Green Lake plutons intrude metavolcanics with ages of 177-220 Ma. Pluton emplacement pressure lowers with age, including 3.2 kb (12 km) at 731°C for the Green Lake pluton and 2.3-2.4 kb (9 km) at ~696 °C for the Soldier Lake and the Cathedral Peak.

In the Saddlebag Lake area, the 221 Ma Saddlebag quartz diorite intrudes metavolcanics of the Koip sequence with ages of 232 Ma and younger.

Host units achieved peak mid- to high-amphibolite grade near plutons based on mineral assemblages. We are currently collecting plutonic and metamorphic P-T data for the Saddlebag area, but for Cinko and Soldier lake regions, hornblende-plagioclase thermometry yields metamorphic temperatures of 640-717°C, exhibiting near thermal equilibration with adjacent plutons and geothermal gradients in excess of 70°C/km. In areas, host units descended to depths of 11 or greater kilometers prior to or during subsequent pluton emplacement.

Others have suggested downward flow of host rock during rise of magma in the central (Tobisch et al., 2000) and southern (Saleeby, 1990; Saleeby et al., 2007) Sierras suggesting that this may be a major, Sierra-wide event. We therefore propose that this regional downward flow may be a batholith-scale geodynamic process in response to magma surges in arcs.