Crustal Evolution and Global Tectonics: A Petrogenic View

Studies of the relative abundances and characteristics of rock types, series, and complexes through time reflect both general and subtle features of crustal and upper-mantle environments. These studies also suggest salient aspects of the dynamothermal and global tectonic history of the last 3,500 m.y. Among the many petrochemical indices, the ratio K 2 O/Na 2 O reflects both the degree of differentiation (fractionation) of igneous rocks and the more or less mature (residuate) nature of clastic sediments. Accordingly, this ratio is a guide to the thickness, composition, and stability of source and crusial site of rock emplacement. Average K 2 O/Na 2 O of rocks, rock complexes, and terranes decreasing from 1 are typical of rock series formed in primitive borderland and arc-to–oceanic-crustal sites. Average values of K 2 O/Na 2 O increasing from 1 characterize rock series evolving in and on more mature arc-to-continental sites. Relative abundances of the most common and characteristic rock assemblages of various ages and their weighted average K 2 O/Na 2 O suggest profound episodicity in crustal evolution and global tectonics. At least three macro-episodes of major significance are defined: the Archean, >2,500 m.y. B.P.; the Proterozoic-Paleozoic, 250 m.y. B.P.; and the Mesozoic-Cenozoic, ∼250 m.y. B.P. to present. Within these macro-episodes, there are innumerable subordinate episodes and variously developed rock cycles. Most major Archean rocks, rock complexes, and terranes have a K 2 O/Na 2 O of Archean thermal gradients varied abruptly both vertically and laterally from very steep to moderate and created highly unstable, thin to thick, labile lithosphere and protocrusts commonly inhospitable to the evolution of widespread, highly fractionated calc-alkaline series or mature sediments. Heat transfer was largely via convection and advection. The advective loss was undoubtedly large, associated especially with the extrusion of floods of ultramafic to felsic magmas and related upward streaming of volatiles. Culmination of major Archean orogenies and subsequent thermal decay about 2,500 m.y. B.P. induced relative crustal quiet and resetting of most Archean Rb/Sr and K/Ar radiomentric clocks between 2,600 and 2,400 m.y. B.P. In the early Proterozoic (∼1,700 to 2,300 m.y. B.P.), many segments of the megacontinent(s) were sufficiently cool, thickened, and fractionated to remain quasi-coherent, deforming and cracking internally as well as marginally above convecting mantle forces. Widespread, relatively ensialic orogenic regions evolved above many thinner sialic zones. These were repeatedly refractionated and redated as they became populated by igneous series with K 2 O/Na 2 O commonly 2 O/Na 2 O as high as 2 or more. Sedimentary K 2 O/Na 2 O increased much faster than did the igneous ratio, as blankets, basins, and prisms of more mature, frequently recycled sediments formed on all the continents. Two of the several major Proterozoic orogenic episodes culminated at about 1,700 m.y. B.P. and 1,000 m.y. B.P. The latter, often called the “Greenville Event,” between 1,000 and 1,200 m.y. B.P., is characterized by the evolution of both K- and Na-rich terranes and distinctive granulite and anorthositic-charnokitic complexes. Most of these Grenville rock complexes evolved between and on relatively thicker, more fractionated continental crust than did the Archean granulites, as indicated by the relative abundance of less telescoped prograde terranes. The Grenville thermal pulses and decay involving much advective and convective heat loss again reset numerous Rb/Sr and K/Ar ages of rocks emplaced 100 to 2,000 m.y. earlier. A post-Grenville relatively amagmatic period of 400 to 500 m.y. after 1,000 m.y. B.P., preceded the incipient rifting of large continental segments as the great Pan–African-Appalachian-Her-cyian-Caledonian orogenies waxed. But large-scale drift of most of these continental segments seems precluded by the continuing alignment of the older, including Archean, fold belts and by the predominantly continental rock component emplaced in most Proterozoic and Paleozoic orogens. Weighted average K 2 O/Na 2 O of Proterozoic igneous complexes are commonly > 1.2 and, in sedimentary sequences, often > 2. Nevertheless, the occurrences of lower Paleozoic ophiolite, blueschist, and less fractionated rock complexes in many Phanerozoic orogens suggest the onset of the large-scale continental rifting and global drift characteristic of post-Permian time. These events are reflected in variously depressed lower Paleozoic K 2 O/Na 2 O and, subsequently, in the plunge of average K 2 O/Na 2 O of major Mesozoic rock complexes to near Archean lows of ∼0.5 to 0.9, especially in the circum-Pacific. The petrochemical and structural data indicate the significant differences as well as similarities between the Archean and post-Permian. The Archean was dominated by the evolution and aggregation of protocontinents, arcs, and intervening oceanic crusts. The post-Permian was dominated by the unique fragmentation and widespread drift (>1,500 km) of large, thick, cool, fractionated, continental fragments and by the birth of new large ocean basins and island-arc chains.