Sanukitoid crystallization relations at 1.0 and 0.3 GPa

Abstract

Sanukitoid (vaugnerites, appinites, durbachites) intrusions have been described in multiple locations around the world. Initially thought as an only basic-intermediate series, increasing interest in these rocks have pointed to the conclusion that they represent a whole differentiation series. A distinct geochemistry with high MgO, LILE and LREE (specially K, Ba and Sr) and depletion in CaO differentiates them from the standard calc-alkaline batholiths they appear associated with. While some of such features may be inherited from the metasomatized mantle source, no consistent explanation has been given for the singular geochemistry of the series. Accordingly, we propose two initial hypotheses: (1) the sanukitoid array is due to the existence of a cotectic line that determines the behaviour of the system; (2) the sanukitoid array does not represent a cotectic series, but the result of a ubiquitous process that deviates the rocks from their ideal differentiation trend. To test the thermodynamic behaviour of the system piston cylinder experiments were conducted at different temperatures in two pressure series appropriate for lower and upper crust differentiation (1.0, 0.3 GPa and 1100 °C, 1050 °C, 1000 °C, 900 °C). Two starting compositions are used: a sanukitoid enclave from Los Pedroches batholith (SW Spain) and a hydrous synthetic glass. Experimental results show that, while 1.0 GPa runs follow the calc-alkaline cotectic, 0.3 GPa runs follow a line that coincides with the sanukitoid series, proving the existence of a low pressure cotectic line. However, lack of cumulates at shallow depths and low water contents in the fractionated magmas (the granites) suggest that a complementary mechanism must be involved. The observed geochemical features may also be accounted for by contamination by entrainment of orthopyroxene from the source or early magmatic cumulates. This is supported by the showcased crystallization pattern, with orthopyroxene representing the liquidus phase, the presence of amphibole clots in these rocks, and the presence of autoliths with abundant orthopyroxene in sanukitoid intrusions. Additionally, the existence of a low pressure cotectic system induces the re-equilibration and digestion of such entrained orthopyroxene in the liquid when reaching shallow depths, leaving a seemingly homogeneous liquid. The two discussed mechanisms are valid, compatible, complementary and account for the geochemical trend shown by the series.