MODAL, TEXTURAL ANALYSIS AND GEOCHEMICAL SIGNATURE OF INTRUSIVE IGNEOUS ROCKS IN A MINGLING ENVIRONMENT AT SALEM WILLOWS, MA

This study focuses on the variation of mode, texture and geochemical signature of heterogeneous igneous intrusive rocks found at the contact between felsic and mafic melts intruded within the West Avalon terrane at Salem Willows (MA). Here, the mafic unit is generally interpreted as the Salem Gabbro-Diorite (~424 Ma), while the felsic unit is commonly assigned to the Beverly Syenite (~425 Ma).

A felsic sample and two mafic samples were selected to represent extreme examples of the various composition and texture observed along the outcrop. The samples were analyzed using a petrographic microscope and Adobe Photoshop to obtain modal analyses. Grain size distributions were determined as averages of the long and short axis of randomly selected grains. The modes were used to assign rock names based on the QAPF Igneous classification. Additionally, we conducted major, trace element and radiogenic isotope (Pb-Nd-Sr-Hf) analyses on the same set of samples.

The mode, texture and geochemical signature in the rocks analyzed show changes at a centimeter scale. The first mafic rock, classified as a porphyritic tonalite, contains elongated plagioclase phenocrysts dispersed in a matrix of hornblende, biotite and minor amounts of quartz and pyroxene. The euhedral plagioclase phenocrysts exhibit a variety of disequilibrium textures which include a combination of complex zoning patterns and resorption. The second mafic sample collected only a few centimeters away, is quite different. This rock has been classified as a pyroxene-hornblende gabbro composed of plagioclase, amphibole, biotite, clinopyroxene and minor amounts of olivine. This sample exhibits a poikilitic phaneritic texture with abundant anhedral grains. The felsic sample is enriched in alkali feldspar and plagioclase with minor amounts of quartz, biotite and amphibole. This is a syenite with an equigranular granoblastic texture. The boundary between the mafic and felsic unit shows interfingering and lobate contacts where the felsic unit is present all-around mafic enclaves. These relationships suggest that the formation of these rocks is best explained by a process of magma mingling presumably occurring due to synchronous emplacement of felsic and mafic magmas, which is consistent with the similarity in age of these magmatic suites.