Three volcano-sedimentary sequences and related intrusive suites, ranging in age from pre-700 Ma to about 570 Ma, are recorded in the Midyan region of northwestern Saudi Arabia. Crustal history involved island-arc accretion and compressional tectonics up to about 625 Ma and extensional tectonics and rifting after that date. Possible correlatives of the Midyan stratigraphy, as well as ophiolitic rocks of a probable suture zone, may exist in southeastern Sinai, although doubt exists because of the conflicting views of workers in that area.
Summary The amphibolites and mafic schists that occur in the Precambrian metamorphic rocks of the Grand Canyon are divided into five major groups: anthophyllite and cordierite-anthophyllite rocks, early amphibolites, Granite Park mafic complex, hornblende-beating dykes, and tremolite-bearing dykes. Many types of amphibole occur in these rocks. Microprobe analyses identify gedrite, anthophyllite, cummingtonite, and grunerite, as well as three groups of calcic amphiboles. These last range in composition from colourless tremo-litic or actinolitic amphiboles, through pale-green horn-blende, to strongly pleochroic green-brown hornblende, which contains a significant proportion of the tscherma-kite and pargasite endmembers. Phase relationships between the coexisting amphiboles and other minerals are presented for two regional metamorphic events. Assem-blages containing chlorite, garnet, and hornblende were formed during the first event; from the absence of staurolite, but the presence of almandine garnet and oligoclase-andesine, it is concluded that the metamorphic grade was between the upper greenschist and the lower amphibolite facies. The second period of metamorphism produced rocks of the staurolite and sillimanite zones, within which three main ‘subfacies’ can be distinguished on the basis of phase relationships in the mafic schists.
