Root zone of the Late Proterozoic Salma Caldera, northeastern Arabian Shield, Kingdom of Saudi Arabia

Abstract

The eroded root of the late Proterozoic Salma caldera crops out in a striking, roughly elliptical feature, about 27 km long and 22 km wide, near the northeastern edge of the Arabian Shield. The caldera is genetically part of an elongate alka!ic granitic massif (Jabal Salma) that extends 35 km from the caldera to the southwest. Comenditic ash flow tuff and lava(?) of the caldera fill, probably more than 1 km thick, are the oldest recognized rocks of the caldera complex. These rocks were erupted during caldera collapse associated with the rapid evacuation of the upper, mildly peralkalic part of a zoned magma reservoir. Within the caldera fill, a massive, lithic-rich intracaldera rhyolite, probably a lava in excess of 1 km thick, is overlain by a layered ash flow sequence. Numerous megabreccia blocks, probably derived from the caldera wall, occur in the massive rhyolite. Open folds in the layered volcanic rocks may be due to high-temperature slumping of the rocks toward the center of the caldera following collapse. Later peralkalic granite that intruded the caldera ring fracture zone occurs in an arcuate pattern outside the area of exposed caldera fill. After caldera collapse, metaluminous to peraluminous magma rose beneath the caldera at approximately 580 Ma and solidified as biotite alkali granite, rim syenogranite, and late, high-level granophyre. Rare earth element abundances indicate that the layered rhyolite tuff, peralkalic granite, and granophyre are chemically more evolved than the biotite alkali granite and rim syenogranite. The granophyre intruded the caldera fill as a dome-shaped body composed of numerous sheetlike masses. Granophyric texture resulted from rapid pressure release and quenching accompanying the intrusion of each sheet. Maximum penetration of the granophyre into overlying rocks occurred in the central region and along the west side of the caldera, where the caldera fill volcanic rocks have been removed by erosion. No apparent structural doming of the exposed volcanic rocks along the east side of the caldera took place; the layered ash flows commonly dip steeply toward the center of the caldera. Postemplacement deformation and metamorphism of the caldera are minimal. Small-displacement strikeslip faults cut the complex, which is tilted to the northeast by no more than about 2 ø .

Active tectonic and magmatic processes beneath Long Valley Caldera, eastern California: An overview

“…In both the Questa and Salma calderas, repeated intrusions of dikes into the shallow crust persisted for several million years after caldera collapse. A few of these many post caldera dikes reached the surface to produce eruptions [Lipman, 1983;Kellogg, [Bailey, 1982[Bailey, , 1984Kissling et al, 1984]. These midcrustal bodies may represent the zone in which mafic magmas from mantle sources leak into the lower crust along elongate bands controlled by regional extensional stresses.…”

A gravity survey of parts of quadrangles 26E, 26F, 27E, and 27F, northeastern Arabian Shield, Kingdom of Saudi Arabia

“…This low is associated with relatively young plutonic rocks. The low appeals to be shifted somewhat south-east from where the center of the caldera is indicated to be, and over the southern lobe of the Salma pluton, judging by the annular outcrops (Kellogg, 1985).…”

“…The last episode of plutonism was essentially post-orogenic (about 610-570 Ma). Caldera-derived volcanic rocks are also associated with this period (Kellogg, 1985), during which time the cratonization of the Shield was completed. The composition of these later plutons is generally felsic (granitic and granodioritic) and chemically highly evolved.…”