Deformation lamellae oriented parallel to the (1013) and (0001) crystallographic planes in quartz have been considered to be deformation structures unique to shock metamorphism induced by meteorite impact. Rocks of the Belt Supergroup of the Coeur d'Alene district, Idaho, contain quartz with deformation lamellae parallel to both (1013) and (0001). All available evidence indicates a geotectonic rather than an astrotectonic origin for the deformation lamellae of the district. Therefore, the uniqueness of these orientations to astrotectonic deformation is doubtful.Recently there has been considerable interest in establishing petrographic criteria for the recognition of structures formed by meteorite impact. Experimental deformation of rock specimens by hypervelocity shock waves has produced what are considered to be unique indicators of shock deformation (1). Among these unique indicators are multiple sets of planar lamellae in quartz, with preferred orientation parallel to (1013) and, less strongly, parallel to (0001) crystallographic planes. The uniqueness of these deformation structures is (based largely on the failure to produce lamellae of these orientations in experimental deformation of quartz under low rates of strain and on the absence of reports of similar orientations from geotectonically deformed rocks (2).Accepting the uniqueness of (0001) and (1013) lamellae, French (1) argues that their presence in quartz of the Onaping Formation at Sudbury, Ontario, is evidence that the Onaping was deposited immediately after a meteorite impact that formed the Sudbury basin. Abstract. Deformation lamellae oriented parallel to the (1013) and (0001) crystallographic planes in quartz have been considered to be deformation structures unique to shock metamorphism induced by meteorite impact. Rocks of the Belt Supergroup of the Coeur d'Alene district, Idaho, contain quartz with deformation lamellae parallel to both (1013) and (0001).All available evidence indicates a geotectonic rather than an astrotectonic origin for the deformation lamellae of the district. Therefore, the uniqueness of these orientations to astrotectonic deformation is doubtful.Recently there has been considerable interest in establishing petrographic criteria for the recognition of structures formed by meteorite impact. Experimental deformation of rock specimens by hypervelocity shock waves has produced what are considered to be unique indicators of shock deformation (1). Among these unique indicators are multiple sets of planar lamellae in quartz, with preferred orientation parallel to (1013) and, less strongly, parallel to (0001) crystallographic planes. The uniqueness of these deformation structures is (based largely on the failure to produce lamellae of these orientations in experimental deformation of quartz under low rates of strain and on the absence of reports of similar orientations from geotectonically deformed rocks (2).Accepting the uniqueness of (0001) and (1013) lamellae, French (1) argues that their presence in quartz of the Onaping Forma...

Early cratonal development of the Arabian Shield of southwestern Saudi Arabia began with the deposition of calcic to calc-alkalic, basaltic to dacitic volcanic rocks, and immature sedimentary rocks that subsequently were moderately deformed, metamorphosed, and intruded about 960 Ma ago by dioritic batholiths of mantle derivation (87Sr/86Sr = 0.7029). A thick sequence of calc-alkalic andesitic to rhyodacitic volcanic rocks and volcanoclastic wackes was deposited unconformably on this neocraton. Regional greenschistfacies metamorphism, intensive deformation along north-trending structures, and intrusion of mantle-derived (87Sr/86Sr = 0.7028) dioritic to granodioritic batholiths occurred about 800 Ma. Granodiorite was emplaced as injection gneiss about 785 Ma (87Sr/86Sr = 0.7028- 0.7035) in localized areas of gneiss doming and amphibolite to granulite facies metamorphism. Deposition of clastic and volcanic rocks overlapped in time and followed orogeny at 785 Ma. These deposits, together with the older rocks, were deformed, metamorphosed to greenschist facies, and intruded by calc-alkalic plutons (87Sr/86Sr = 0.7035) between 600 and 650 Ma. Late cratonal development between 570 and 550 Ma involved moderate pulses of volcanism, deformation, metamorphism to greenschist facies, and intrusion of quartz monzonite and granite. Cratonization appears to have evolved in an intraoceanic, island-arc environment of comagmatic volcanism and intrusion.

9 15 Results of rubidium-strontium analyses-Continued Andesitic assemblage ____-_-_________ Undisturbed isotopic systems _____________ Disturbed isotopic systems ________________ Foliated diorite to trondhjemite batholiths _____ Granodiorite gneiss domes ___________________ Late-orogenic or postorogenic granodiorite to

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Two main subdivisions of layered rocks are recognized in the southern Arabian Shield south of lat 22° N. These are an older ensimatic-arc complex, which formed 1100-800 m.y. ago, and a younger marginal-arc complex, which formed 800-690 m.y. ago. The older ensimatic-arc complex, located in the southwestern part of the Shield, includes graywacke and mafic to intermediate volcanic rocks of the essentially contemporaneous Baish, Bahah, and Jiddah groups.Although the younger arc complex is also dominantly ensimatic in character, it is also partly superimposed over the older ensimaticarc complex.The superimposed portions of the younger arc complex are represented by the Ablah, Samran, and possibly the Arafat groups. The ensimatic portion of the younger arc group is represented by the Halaban group, which was deposited to the east and northeast of the older ensimatic-arc complex.The Halaban group includes andesitic and dacitic volcanic rocks and associated clastic sedimentary rocks. The layered rocks of both arc complexes are intruded by dioritic (quartz diorite, tonalite, trondhjemite) plutonic rocks.The southern Shield is also subdivided into a number of structurally bounded, north-trending tectonic belts. Within the older ensimatic complex, three belts are recognized. From west to east, these are the Lith, Bidah, and Tayyah belts. Within these three belts, progressive facies changes indicate a gradation from deep-water facies in the south to shallow-water or terrestrial facies in the north.The distribution of dioritic batholiths, as well as the distribution of layered-rock facies, suggests a northwesttrending axis for the older ensimatic-arc complex.The younger arc complex is present within six belts, the Makkah T7U.S. Geological Survey,