ABSTRACT Ferroan granite is a characteristic rock type of the Laurentian margin. It is commonly associated with anorthosite and related rocks. Ferroan granites are strongly enriched in iron, are alkalic to alkali-calcic, and are generally metaluminous. These geochemical characteristics reflect their tholeiitic parental magma source and relatively reducing and anhydrous conditions of crystallization. Their compositions distinguish them from arc magmas, which are magnesian and calcic to calc-alkalic. Ferroan granite magmas are hot, which promotes partial melting of their crustal wall rocks. Assimilation of these silica-rich and peraluminous melts drives the resulting magmas to higher silica and aluminum saturation values. Where Proterozoic ferroan granites intrude Archean crust, their mantle component is readily identified isotopically, but this is more difficult where they intrude relatively juvenile crust. Ferroan granite forms in tectonic environments that allow partial melts of tholeiitic mantle to pond and differentiate at or near the base of the crust. Phanerozoic examples occur in plume settings, such as the Snake River Plain and Yellowstone, or under certain conditions involving slab rollback, such as those that formed the Cenozoic topaz rhyolites of the western United States or ferroan rhyolites of the Sierra Madre Occidental. It is possible that the long-lived supercontinent Nuna-Rodinia, of which Laurentia was a part, formed an insulating lid that raised underlying mantle temperatures and created a unique environment that enabled emplacement of large volumes of mafic melt at the base of the crust. Ascent of felsic differentiates accompanied by variable crustal assimilation may have created large volumes of Proterozoic ferroan granite and related rocks.

ABSTRACT Mesoproterozoic crust is widely exposed in the Grenville Province portion of northeastern Laurentia, where it is interpreted as an assemblage of two continental-arc segments separated by a composite arc belt (Quebecia) with island-arc remnants. A synthesis of the geologic context, types, and geochemical patterns of 1.5–1.35 Ga granitoids reveals a regional distribution in each segment, with dioritic to granitic plutonism variably associated with arc-related volcano-sedimentary belts in the south and inboard monzonitic to granitic plutonism in the north. In addition, belts of dioritic to granitic orthogneisses occupy intermediate positions in Quebecia and in the west. The inboard granites are consistently old in all segments (1.5–1.45 Ga), but the preserved volcano-sedimentary belts are older in the east and in Quebecia (1.5–1.45 Ga) and younger in the west (1.39? and 1.36 Ga), while the belts of orthogneisses show a large spread of ages at 1.45–1.37 Ga. Granitoids in the volcano-sedimentary belts and the orthogneisses include magnesian, calcic to calc-alkalic components to ferroan, alkali-calcic components. In contrast, the inboard plutons are dominantly ferroan and alkali-calcic to alkalic in the continental-arc segments, where they are locally associated with anorthosite-mangerite-charnockite-granite (AMCG) suites. Collectively, the different types of granitoid magmatism can be linked to an active margin, with subduction under northeastern Laurentia, involving arc building, arc rifting, back-arc opening and inboard extension, and amalgamation processes variably operating at different parts of the margin and at different times. In addition, the data provide a basis for comparison with other parts of the eastern to southwestern Laurentian margin in the 1.5–1.35 Ga time frame.

Field evidence, as well as binary and ternary variation diagrams, provide evidence that the anorthosite-mangerite-charnockite-granite gneiss (AMCG) suite of the Adirondacks evolved in bimodal magmatic complexes without a continuous line of liquid descent from mafic through acidic igneous members. Major- and trace-element distributions suggest an anorogenic setting for the emplacement of AMCG magmas. Recent U-Pb zircon dating indicates that AMCG complexes were emplaced during the interval 1160 to 1130 Ma. Peak granulite facies conditions appear to have occurred at ~1070 to 1030 Ma, as indicated by metamorphic zircons formed by exsolution of Zr from mafic silicates and Fe, Ti oxides, as well as by monazite and sphene cooling ages. Continued exsolution of Zr from magnetite-ilmenite that reequilibrated to 500° to 600°C resulted in zircon ages of ~ 1000 Ma in oxide-rich rocks. The AMCG suite of the Adirondacks appears to be part of a protracted anorogenic cycle that traversed North America and northern Europe in mid-Proterozoic time and manifested the break-up of a supercontinent believed to have been assembled from 1900 to 1600 Ma.

The Roseland district of Nelson and Amherst Counties, Virginia, is a typical Grenvillian terrane, analogous to similar terranes of eastern Canada. The oldest rocks in the Roseland district are layered granulites, quartz mangerites, and quartzo-feldspathic gneisses. Ages on zircon from the Shaeffer Hollow Granite, a leucocratic granite related to these oldest rock types, are discordant but apparently pre-Grenvillian. The Roseland Anorthosite intrudes these older rocks and consists of andesine antiperthite megacrysts and blue quartz in a finer grained oligoclase-K feldspar matrix. The Roseland Anorthosite is more alkalic and silicic than massif anorthosites elsewhere. Pyroxene megacrysts and rutile are found in its border areas. After the emplacement of the anorthosite, the Roses Mill and Turkey Mountain ferrodiorite-charnockite plutons (of the Roses Mill Plutonic Suite) were intruded. Layered diorite and nelsonite, an ilmenite-apatite rock, are found near the bases of these plutons. These rocks may have formed, in part, by liquid immiscibility. Ages determined on the Roses Mill Pluton are about 970 m.y. The largest part of the plutons is altered to biotitic granitic augen gneiss. The Rockfish Valley deformation zone crosses the district from northeast to southwest. Northwest of the deformation zone are ferrodioritic charnockitic rocks of the Pedlar massif, which are slightly older than the Roses Mill Pluton. These ages may have been modified by metamorphism. The Mobley Mountain Granite is a fine- to medium-grained, subsolvus two mica granite and has been dated at 650 m.y. Various mafic and ultramafic dikes, of different ages, are present throughout the district. The major structure of the district is the Roseland dome, cored by the Roseland Anorthosite, and trending northeast for at least 22 km. Three periods of deformation are evident; one is possibly pre-Grenvillian and is seen only in the oldest rocks. The Roses Mill Plutonic Suite was deformed and underwent retrograde metamorphism to lower amphibolite assemblages in Proterozoic Z time. Paleozoic deformation was responsible for a reactivation of the Rockfish Valley deformation zone, which originated as a Precambrian feature and a selective overprinting of retrograde greenschist facies metamorphism on the granulite- to upper amphibolite-facies assemblages of the country rock. The principal resources of the district are rutile and ilmenite. Both are present as hard rock and saprolite deposits. The rutile formed at the anorthosite border, and ilmenite is contained largely in nelsonite and mafic ferrodiorite bodies.