An ion and electron microprobe study of the
mineralogy of enclaves and host syenites of the Red Hill
Complex, New Hampshire, U.S.A.
The Red Hill complex of New Hampshire is unusual for the White Mountain Magma Series of northern New England because it consists of both silica-undersaturated and - saturated to -oversaturated syenites. Amphibole, pyroxene, and apatite in two of the saturated units, the Outer Coarse Syenite (OCS) and the Garland Peak Syenite (GPS), and in the undersaturated nepheline Sodalite Syenite (NSS), were analyzed to determine the relationship between coexisting undersaturated and saturated magmas. Mafic enclaves in the NSS and the GPS were also studied to elucidate their relationships with the host syenites. In addition to mafic enclaves, the NSS contains later emplaced camptonitic dikes and associated pipe-like benmoreites. The benmoreites contain amphibole that is compositionally continuous with amphibole in the NSS. However, REE and other trace element abundances in apatite from the benmoreites and the NSS are not compatible with a genetic relationship between the two. Mafic enclaves within the NSS contain amphibole and pyroxene that are compositionally continuous with the NSS. Bulk-rock compositions of the enclaves plot along trends defined by the NSS. Furthermore, chondrite-normalized REE patterns for apatite in both the enclaves and the NSS are parallel, and REE abundances increase systematically for the enclaves to the NSS. We therefore suggest that the enclaves represent magmas similar to the NSS parent that intruded up into its daughter products. These magmas appear to have been tephritic to phonotephritic in composition. Abundances of REE in apatite in the Nepheline Sodalite Syenite (NSS) are distinct from those in apatite in the silica-saturated OCS. OCS apatites have LREE abundances up to 26,000 times chondrites and La/Yb ratios of 16-27. NSS apatites have comparable LREE concentrations, but HREE abundances are considerably lower than those of the OCS; La/Yb ratios range from 68 to 104. These observed differences in both the REE and other trace element abundances between apatite in the two rocks present difficulties with a common parental magma hypothesis for the NSS and OCS. Hence it is suggested that, although the OCS and NSS are contemporaneous in time and space, they are probably not consanguineous. The silica-saturated GPS is a fine-grained syenite containing strongly zoned amphiboles with kaersutite to hastingsite cores rimmed by hastingsitic hornblende and ferro- hornblende. Discrete grains of hastingsitic hornblende and ferro-hornblende occur in a feldspar-quartz groundmass. Coarser-grained, quartz-rich patches, containing feldspars and ferro-hornblende and ferrendenite, are also found in the GPS. The kaersutite cores are identical to the amphibole in the GPS enclaves and the NSS suite. These GPS enclaves are silica undersaturated; the kaersutite cores in the GPS host rocks are probably xenocrysts derived from disaggregated undersaturated magmas similar to that represented by the enclaves.
Dorais and Floss (1992) J. Petrol. 33, 1193-1218.
Last revised: 12-2003