Direct dating of Adirondack massif anorthosite by U-Pb SHRIMP analysis of igneous zircon: Implications for AMCG complexes
The low abundance of igneous zircon in Proterozoic massif anorthosites has presented a major obstacle to the acquisition of direct absolute ages of crystallization for these important rocks. Indirect dating that relies on zircon ages from associated mangerite-charnockite-granite granitoids assumes that they have a coeval relationship with anorthosite that requires documentation. SHRIMP (sensitive, high-resolution ion-microprobe) U-Pb zircon-dating techniques provide a powerful means for directly dating the small populations of zircons in anorthositic rocks and for resolving problems with inheritance. Within the Adirondack Mountains, 10 samples of massif anorthosite have yielded more-than-sufficient quantities of igneous zircon to establish directly the ages of the region9s classic anorthosite occurrences (e.g., the Marcy and Oregon Dome massifs). In addition, a ferrogabbro, a ferrodiorite, and a coronitic olivine metagabbro, all crosscutting massif anorthosite, were dated. The average age of this suite of 13 anorthositic samples is 1154 ± 6 Ma (MSWD [mean square of weighted deviates] = 0.26, probability = 0.99). In addition, eight associated granitoids have been dated by SHRIMP techniques and complement another five previously dated by multi-grain thermal-ionization mass spectrometry (TIMS) methods. The 13 granitoids yield an average age of 1158 ± 5 (MSWD = 0.89, probability = 0.60) and are broadly coeval with the massif anorthosite. The overlapping ages provide evidence that these rocks constitute a single, composite anorthosite-mangerite-charnockite-granite (AMCG) suite intruded at ca. 1155 Ma, an age corresponding to the ages of major AMCG suites in the Grenville province in Canada (e.g., Morin and Lac St-Jean). Although rocks of the Adirondack AMCG suite are now documented as broadly coeval, it does not follow that the constituent AMCG lithologies were comagmatic. Field relationships and mineral disequilibria in transitional zones are inconsistent with derivation from a single parental magma. Moreover, the presence of older (ca. 1.2–1.3 Ga) inherited cores in some zircons from AMCG granitoids conflicts with derivation of these rocks from magmas that formed anorthosite, gabbro, or ferrodiorite, or jotunite, in which zircons are highly soluble. The slightly older ca. 1158 Ma average age of the mangeritic and charnockitic members of the AMCG suite is consistent with an origin as early lower-crustal anatectites that left behind pyroxene-plagioclase restites. This refractory material then reacted (by assimilation–fractional crystallization [AFC]) with ponded, mantle-derived gabbroic magmas to produce plagioclase-rich crystal mushes with crustal isotopic signatures, as proposed much earlier by R.F. Emslie. These magmas are considered to be parental to the Adirondack anorthosite, and upon ascent they were emplaced in proximity to still hot, earlier mangeritic and charnockitic bodies where they underwent further fractionation. The composite nature of the Marcy massif documents that this process was repeated in several sequential pulses.