FORMATION OF HYBRID MELTS VIA MAGMA MIXING IN THE CAPE ANN PLUTONIC COMPLEX, MASSACHUSETTS USA

Bimodal plutonism is commonly observed in tensional regimes. The injection of mafic magma into a felsic magma chamber can lead to the formation of hybrid melts via magma mixing. The effectiveness of this process can be limited by rheological/thermal constraints on mixing. However, multiple injections of compositionally distinct magmas may significantly alter the trace element and isotopic chemistries of both magmas. To study this, and gabbroic magmas (Andrews Point in Rockport, Massachusetts and Winter Island Park in Salem, Massachusetts). These rocks belong to the Cape Anne Plutonic Complex and were emplaced into the Composite Avalon Terrane during Late Ordovician-Devonian time. At both locations “end member” and intermediate phases were collected for bulk, trace and isotopic chemistries, as well as petrographic thin section analysis. ⁸⁶Sr/⁸⁷Sr ratios for possible end samples were collected from two well-documented locations that show co-mingling between syenitic members range from 0.775 for the syenite samples to 0.703 for the gabbro samples and intermediate sample (co-mingled?) ranges from 0.707 to 0708. Despite the wide range in strontium ratios, the trace element data show remarkable similarities on both REE and Spider plots indicating the trace element chemistries in this system have been significantly altered. These analyses indicate that both physical and diffusional mixing are contributing mechanisms for the formation of intermediate composition magmas with little evidence for crystal fractionation as an important component of magma evolution at these locations. Although difficult to determine due to lack of data and exposure, the final question still remains as to how the magmas were emplaced. The three possibilities include: 1) a partially crystallized granitic magma intruded into a mafic magma chamber, 2) a basaltic sill or sills intrude into a granitic magma chamber or, 3) both the felsic and the mafic magmas were emplaced simultaneously, facilitating efficient mixing/ contamination.