Consistency between apatite and zircon petrochronology supports robustness of apatite in fingerprinting igneous processes in porphyry systems

Abstract

Apatite low-temperature thermochronology can be double or even triple dated allowing for a reconstruction of the thermal history of rock from ~ 550 oC to near-surface temperatures. Even though it has disadvantageous U–Th–Pb contents (high Pb contents and low U and Th contents) and an unstable nature, apatite is still regarded to have the same robustness in fingerprinting igneous processes in porphyry systems as zircon, so far as to be replace zircon. Hence, we systematically studied characteristics of morphology, geochronology and geochemistry of apatite hosted in syenogranite and monzogranite intrusive rocks in the large Hutouya skarn deposit, in order to corroborate its potential thermochronological monitoring capabilities like zircon in fingerprinting igneous processes in porphyry systems. In this study, apatite grains can be subdivided into two types, FI-free Apatite I formed in the early less fractionated magma and FI-rich Apatite II crystallized in the late highly fractionated magma stage. We obtained ages of 229.0 ± 6.6 Ma in syenogranite and 224.3 ± 4.5 Ma / 223.7 ± 3.9 Ma in monzogranite from Apatite I of magmatic origins. Zircon grains in the two granites can be classified into three types. Zircon I is characterized by transparent and bright zones, Zircon II by dark and metamict features, and Zircon III by mineral inclusions. Zircon I grains with a magmatic texture of well-developed bright oscillatory zones, are most likely primary magmatic zircon that crystallized early in the evolution of granitic magma, dating results of which are 224.70 ± 0.61 Ma in syenogranite intrusions and 225.75 ± 0.66 Ma / 226.31 ± 0.78 Ma in monzogranite, respectively. The apatite–zircon timing is coincident. Furthermore, apatite trace rare earth element contents in the syenogranite and monzogranite intrusions display a negative-slope chondrite-normalized distribution from La to Lu with strong negative Eu anomalies and weak positive Ce anomalies, with major element contents that are statistically identical with enriched F but poor Cl. Zircon trace element compositions in the two intrusions show consistent and steeply increasing chondrite-normalized REE diagrams from La to Lu with negative Eu anomalies and strong positive Ce anomalies. Accordingly, apatite U–Pb dates and the corresponding in-situ trace element compositions and isotopes can test precise constraints on rock formation ages, temperature, oxygen fugacity, material source, and tectonic background, which can be relatively more robust when used as proxies for magma oxidation state.