Mechanism of Carbonate Assimilation in the Middle Ordovician Hortavær Igneous Complex, north-central Norway

The Hortavær igneous complex consists primarily of mafic to intermediate magmas emplaced as sheets into marble, calc-silicate, semipelitic, and quartzofeldspathic rocks as well as coeval, cogenetic syenitic magmas. Parental mafic magmas have arc-like trace element signatures and were H2O -rich. Carbonate assimilation was a heterogeneous process and the degree of assimilation varied by locality from 0 to 20 wt percent. Trace element and isotopic compositions of magmatic calcite, compositions of grossular-andradite garnet in endoskarns, and field evidence for partial melting of the endoskarns indicate that carbonate assimilation was a multistage process. Initially, H2O -rich fluid from mafic magmas infiltrated calc-silicate xenoliths/screens, resulting in lowering of their solidus. Partial melting of these screens increased their porosity and decreased their strength, leading to disruption and mingling with mafic magmas as well as bringing carbonate-rich melt into contact with the surrounding silicate magmas. This process also permitted infiltration of silicate melt into the partially melted calc-silicate rocks, enhancing development of garnet ferrohedenbergite ± wollastonite melasyenitic endoskarn. Carbonate melt from the xenoliths mixed with the surrounding mafic magmas by reactive assimilation; this process can sensibly be described as reaction of olivine and calcite components in melt phases to form augitic (fassaitic) clinopyroxene (cpx) and CO2. This enhanced stability of cpx is reflected in the presence of cpx-rich cumulates, fractionation of which resulted in alkali enrichment and formation of abundant syenitic magma. During later stages of magma evolution (monzonitic to syenitic and granitic composition), assimilation of quartzofeldspathic to pelitic rocks became dominant, and magma compositions evolved from nepheline- to quartz-normative. The large amount of host rock (carbonate, calc-silicate or pelite) assimilation in the Hortavær complex was enhanced by the H2O contents of the parental mafic magmas and by the large surface area afforded by multiple injections of these mafic magmas to enclose numerous xenoliths and screens. Contamination of hydrous mafic silicate magmas by the calcite component of calc-silicate rocks might be more common than generally thought. It should be especially effective in situations where basaltic magmas intrude as sheets and physically incorporate large amounts of calcareous host rocks.