MAJOR, TRACE AND RARE EARTH ELEMENT GEOCHEMISTRY OF AMPHIBOLE NEAR THE ROOF OF A MAFIC MAGMA CHAMBER

The ca. 610 Ma Greendale Complex, Avalon terrane, Nova Scotia, is an appinitic suite ranging from ultramafic to felsic in composition intruded during ensialic arc magmatism and crystallized at shallow crustal levels under high pH₂O. Amphibole is the dominant mafic mineral in all rocks and displays extraordinary variability in texture and modal abundance, a characteristic of appinite suites. The sensitivity of amphibole composition (major, trace and REE) to the evolution of water-rich magma is investigated.

All amphiboles in mafic and ultramafic rocks are calcic, with (Ca+Na)_B ≥ 1.34 and Na_B < 0.67 apfu, and Si^IV between 6.1 and 7.3. They range in composition from tschermakite, to tschermakitic hornblende, to magnesio-hornblende and display a dominance of edenite (Na,K_A + Al^IV = Si^IV) substitution. Each sample exhibits remarkably uniform Mg/(Mg+Fe²⁺) over a wide range in Si and the mafic rock amphiboles have lower (0.5 to 0.7) Mg/(Mg+Fe²⁺), compared to the ultramafic rocks (0.7 and 0.9). REE profiles of amphiboles from mafic rocks are all bow-shaped, and are characterized by depletion in LREE (La/Sm ≈ 0.61), a slight depletion in HREE (Gd/Yb ≈ 1.55) and a negative Eu anomaly, which is attributed to co-precipitation of plagioclase. REE profiles of ultramafic amphiboles are divided into two groups: Group A amphiboles occur in all specimens analyzed and are very similar to the profiles of the mafic rocks. In contrast, Group B amphiboles display relative enrichment in light REEs (La/Sm ≈ 2.05), have lower ΣREE, and lack a negative Eu anomaly. Group B amphiboles are more enriched in Th and U and show a more pronounced depletion in Ta, Nb, Ti and Y. Group B amphiboles grew in a reaction relationship with olivine and pyroxene. Groups A and B are virtually indistinguishable with respect to the major elements, suggesting that REE and selected trace elements when combined with textural observations may provide additional insights into crystallization history.