MAFIC ROOTS OF A SATELLITE CONE, EASTERN RING DIKE COMPLEX, WESTERN SIERRA NEVADA, CA: EMPLACEMENT OF THE AUCKLAND GABBRO BY FORCEFUL INTRUSION ALONG AN EARLY CRETACEOUS UNCONFORMITY

The ~5-mi2 Auckland gabbro is interpreted as preserving the roots of a mafic satellite cone developed on the northern margin of a 14-km-diameter Early Cretaceous stratovolcano. Three distinct rock assemblages surround the gabbroic pluton: to the southeast, the gabbroic-to-granitic Eastern Ring Complex (ERC); to the west, the nonmarine Early Cretaceous Goldstein Peak (GP) section; to the north and east, marine rocks of the Jurassic-Triassic Kings Sequence comprising the Lake Kaweah (LK) pendant. The GP and LK sections are in depositional contact immediately northwest of the Auckland gabbro.

The Auckland gabbro is composed of pyroxene- and hornblende-rich magmas comingled at a variety of scales. Unlike other gabbro concentrations in the Stokes Mountain region, no mafic cumulates appear to be present, except for a single occurrence of comb-layered hornblende gabbro. Rafts of Kings Sequence rocks are radially distributed within the northwest quadrant of the pluton. Discontinuous slivers of mylonitic tonalite occur along the northern and western boundaries between the gabbro and the adjacent metamorphic rocks.

Satellite cone growth appears to have occurred by forceful intrusion along the depositional contact between the Early Cretaceous and Jurassic-Triassic metamorphic sections. Field relations support the following emplacement model: (1) intrusion of tonalitic to granodioritic dikes along and sub-parallel to the GP-LK depositional contact, possibly emanating from compositionally similar ring dikes within the ERC; (2) crustal tumescence accommodated by shear deformation within the hot dikes first produced a mylonitic fabric then led to the dikes' ultimate dismemberment; (3) gabbro emplacement and mingling within the opening created by counterclockwise rotation of the LK relative to the GP, as evidenced by the orientations of LK rafts within the gabbros. Secondary deformation in the GP may have formed as magmas intruded up and along the shear zone, squirting out to produce a subhorizontal foliation overprint. Regional considerations support emplacement of the Auckland satellite cone and neighboring ring dike complexes into a NW-trending transtensional-transpressional system, active approximately 125-115 Ma.