The relative contribution of mantle and crustal sources to primitive arc magmas from the Early Palaeozoic Famatinian - Puna arc

OTAMENDI J. E:

Congreso; XVI Congreso Geologico Chileno; 2023

This contribution usedthe composition of primitive mafic rocks within an ancient Early Palaeozoicsubduction-related magmatic arc. The goal is to assess the petrologicalprocesses involved in the cycling of chemical elements from the Earth´s surfacethrough the slab and mantle reservoirs back to the crustal arc magmaticsystems. Based on published data, I compiled geochemical data from mafic andultramafic rocks, ranging from deep crustal cumulates and plutonic rocks toshallow crustal sub-volcanic andvolcanic rocks taken streching along the paleo-are axis between latitudes 22ºand 33º South. A total of about 200 data of bulk rock composition filtered toidentify the mafic rocks better corresponding primitive arc melts. Mainselection criteria were: 1)MgO that varies between 8.0and 12.5 wt%, with SiO2 that varies between 47.5 and 52.5% and TiO2 < 1.5%;2) in addition the Mg# ratio > 0.55, 3) the concentration of Ni between 90and 270 ppm and that of Cr < 1700 ppm; 4) display neutral Eu anomalymeasured as EuN /Eu* between 0.7 and 1.3 (where Eu* = (SmN x GdN)1/2, an Ndenotes normalized to chondrite) and a DyN/GdN ratio < 1.5. Toghether, thesecritieria eliminate rocks that have experience execive early crystal fractionalcrystallization or are liquid-rich differentiated. Using the fielterepopulation of 35 primitive mafic, Iconducted inverse modelling techniques utilizing the composition of primitivemafic rocks that allowed estimating the composition of the ambient mantlesource, which appears to be largely depleted. Furthermore, the inverse approachsuggests that fluid-mobile elements (Th, U, La, and Sr) originate primarilyfrom the slab contribution, indicating that subduction has significant controlover the abundances of Nb and Ta in the modified mantle source. However, it isworth noting that available mineral/melt partition coefficients used in inversemodelling may overestimate the contribution of Sr and Nb from the slab. On theother hand, forward modelling is limited by the incomplete preservation ofancient volcanic arc geology. Nevertheless, forward mixing models that considerthe interaction between slab-released sediment-derived melts and the ambientdepleted mantle can explain the observed variations in Nd and Sr isotopicratios in primitive mafic rocks. By introducing a small mass fraction (1%) ofaverage Cambrian continental sediments subducted beneath the Early Ordovicianarc into a MORB-depleted mantle source, the parental primitive mafic magmasdisplay the abundance of sedimentary-supplied elements such as Th, La, and Ba,as well as radiogenic isotope ratios of 143Nd/144N and 87Sr/86Sr. The chemicaland isotopic composition of deep-seated primitive mafic rocks reveals adecoupling between incompatible trace elements, which shows minimal variation,and radiogenic isotopic ratios, which exhibit a wide range and reflectcontributions from crustal reservoirs. I propose a simple model involvingsimultaneous bulk assimilation (A) and imperfect fractional crystallization(IFC) in an open igneous system to account for the observed variations in traceelements and isotopes, where crystal accumulation and assimilation occurconcurrently. Importantly, a calculated 10% mass fraction input of deep crustalcotamination with supracrustal host rocks that entered into the can obscure theexpected clear signature of sub-arc mantle petrological processes. Clearly, thechemical and isotopic compositions of arc lavas offer a unique opportunity toevaluate contributions from multiple sources within subduction zones. However,distinguishing between two potential reservoirs contributing pre-arccontinental crust to the subduction factory, apart from intra-oceanic volcanicarcs, can complicate the interpretation of geochemical signatures observed involcanic rocks. In conclusion, the assessment of the Famatinian-Puna arc, anancient subduction zone, aligns with estimates from active volcanic arcs,underscoring the general applicability of our findings to subduction zonemagmatism.