Proterozoic anorogenic granite plutonism of North America

Abstract

Anorogenic magnatic activity characterizes much of the late to mid-Proterozoic, from 1030 to 1770 m.y. ago, in a broad belt trending from the southwestern United States, northeastward through Labrador, across southern Greenland, and into the Baltic shield. The association of gabbroic to anorthositic rocks, a separate mangeritic series of primarily intermediate composition, and granite of definite rapakivi affinity comprise an anorogenic “trinity” of world-wide occurrence. With the exception of the 1.76 b.y. Montello batholith (Wisconsin), this episode in North America is restricted to the interval 1.0 to 1.5 b.y. and occurs in three distinct events. Over 70 percent of Proterozoic anorogenic magmatism occurs in a 1.41 to 1.49 b.y. old 600-1000 km wide belt trending from southern California to Labrador that volumetrically and age-wise is totally a North American phenomenon. Renewed anorogenic granite magmatism occurred form 1.34 to 1.41 and from 1.03 to 1.08 b.y. ago in lesser proportions. Although anorthositic and mangeritic rocks are abundant in some provinces (e.g., Labrador), rapakivi granite (in the broad usage of the term) represents by far the most abundant magma-type generated during this nonorogenic period. The modal and mineral composition of these granitic rocks is distinctive and reflects the potassic and iron-enriched character of the magmas and the unique conditions under which crystallization occurred. Principal rock types include biotite ± hornblende granite to adamellite although numerous peraluminous, two-mica (biotite + celadonitic muscovite ± garnet) granites also occur. Crystallization of these epizonal granitic magmas occurred over the range of 640 to 790° C at low total pressures (most less than 2 kb) and at relatively dry conditions. A dramatic difference in crystallization conditions lies with the level of oxygen fugacity which ranges three orders of magnitude from low (ca. QFM) to high (above Ni-NiO), resulting in systematic differences in Fe-Ti oxide mineralogy and mafic silicate composition. Compositionally, the granite magmas are subalkalic and marginally peraluminous (peralkaline varieties are rare to nonexistent). Although some hastingsite or riebeckite-bearing granites may have been derived from fractional crystallization of the mangeritic series, most are primary melts derived from fusion of lower crust material. The high potassium, Fe /Mg, Ba, and rare earth element (REE) composition of the granites is consistent with small degrees of fusion (10-30 percent) of calc-alkaline crust of quartz dioritic, tonalitic, and granodioritic material. Initial Sr isotopic ratios average 0.7051 ± .0025. The relatively low ratios are the result of short residence times (commonly 170 to 340 m.y.) with much of the crustal source being formed in a preceding orogenic event. An earlier melting episode need not have occurred for the source. The dry nature of the magmas is due to vapor under-saturated melting of a metaigneous source with a total water budget less than 1 percent and tied up in relatively stable residual hydrous phases. The derivation of a marginally peraluminous melt from a metaluminous source is a consequence of variable amounts of residual hornblende ± clinopyroxene. The generation of isolated crustal-derived magma under anorogenic conditions is considered to be the result of localized thermal doming in the mantle. The mantle-derived anorthositic and mangeritic magmas may have played an active role in generating the necessary heat of fusion at lower crustal levels. For North America, the 1.4 to 1.5 b.y. event does not have the consistent age progression of a track and is probably an incipient rift that failed to integrate into a world-wide plate system. At a more mature stage, the 1.0 to 1.1 b.y. Keweenawan episode and the midcontinent gravity high represents another unsuccessful rifting attempt of the Proterozoic North American craton. The isolated granite complexes of this age (Pikes Peak, Enchanted Rock, Red Bluff), as well as the earlier, localized 1.76 b.y. and 1.34 to 1.41 b.y. magmatic episodes, may be further representations of a thermal perturbation at mantle depths during this fragile period of crustal “stability.”