Mineralogy and Petrology of the Red Hill Alkaline Igneous Complex, New Hampshire, U.S.A.

The Red Hill intrusion, New Hampshire is one of the alkaline intrusions making up the White Mountain Magma Series. Earlier work has shown that it consists of several units with ring- or plug- like form, in order of intrusion: Outer Coarse Syenite (OCS) plus Nepheline Sodalite Syenite (NSS Fire Tower Sycnite (FTS); Garland Peak Syenite (GPS) Watson Ledge Quartz Syenite (WLQS) Interior Fine Granite (IFG). New studies have revealed trends of increasing alkalinity in both the OCS (OCS-A←OCS-B) and NSS (NSS-A←-B←-C). Conventional K-Ar and Rb-Sr age dating on separated minerals and bulk rocks show that the OCS, NSS, FTS, and GPS have indistinguishable ages at 198.5±1.5 Ma while the IFG formed about 10 Ma later. These values are believed to represent intrusion ages. Amphiboles from the NSS vary from ferro-pargasites through taramites to katophontes and pyroxenes from ferrosalites to aegirine-augites these trends follow the increasing degree of under- saturation in the NSS-A←-C series. The NSS-A contain mafic syenite xenoliths as well as partially resorbcd salitic pyroxene cores within amphiboles. Mineral and isotope data are consistent with these inclusions being cognate suggesting derivation from more-basic parental magmas. Pyroxenes and amphiboles from saturated and oversaturated rocks vary from ferro-salites to ferro-hedenbergites and hastingsites to ferro-edenites, respectively, but OCS-B show fractionation towards aegirine-augites and katophorites. Mineral assemblages crystallized close to the quartz-fayalite-magnetite buffer at a pressure of about 1–1.5 kb under essentially fluid-saturated conditions. It seems likely that the complex formed by emplacement of fractionated magmas derived from a lower-level magma chamber which initially contained a basaltic parent magma. The first pulse of magma had an overall composition similar to OCS plus NSS and was intruded as a sheet-like body which differentiated in situ to give a central unit of highly undersaturated magma (NSS-C). NSS-C samples with relatively radiogenic Sr isotope compositions were modified by introduction of country-rock Sr via circulating fluids. Fractional crystallization of alkali-rich amphiboles from critically undersaturated magma (NSS-A?) in the lower- level chamber led to the development of saturated and oversaturated magmas. These were intruded into the OCS/NSS unit along ring fractures to form the FTS and GPS units. Rock and mineral compositions, including isotope data, are consistent with the IFG forming by partial melting of country rock and being intruded along pre-existing planes of weakness some 10 Ma after the main complex was formed.