Petrogenesis of the post-collisional oxidized felsic potassic rocks: A
case study from the Jianchuan trachytes in eastern Tibetan plateau
Abstract
Petrogenesis of post-collisional felsic potassic rocks and major
controls on their magmatic redox states are still debated. To address
this, we have carried out an integrated petrological–geochemical study
for the trachytes in Jianchuan area, eastern Tibetan plateau. Zircons
from the trachytes yield a U–Pb age of ~35 Ma. These
rocks are characterized by significant LREE and LILE enrichments,
pronounced negative Nb–Ta–Ti–P anomalies and enriched Sr–Nd isotopic
compositions, similar to those of the nearby, coeval mafic potassic
volcanic rocks (e.g., Wozhong shoshonites). The trachytes have zircon
εHf from 0.04 to 1.48 and δ18O from
7.06 to 7.77 ‰, apatite εNd from –3.5 to –2.5, and
clinopyroxene
(87Sr/86Sr)i from
0.70571 to 0.70746. The above-mentioned similarities, together with
uniform whole-rock and mineral isotopes and modeled parental magma
compositions using clinopyroxene trace elements, strongly support the
interpretation that the trachytes were formed by fractional
crystallization of mantle-derived mafic potassic parental melts similar
to the Wozhong shoshonites. Using the amphibole, iron-titanium oxide and
sphene-magnetite-quartz oxybarometers, magmatic oxidation states of the
trachytes are estimated to be at ΔFMQ between 1.49 and 2.86, similar to
the previously reported values for the Wozhong shoshonites. Our results
demonstrate that source is the first-order control on magmatic oxygen
fugacity of the Jianchuan trachytes, implying such mantle-derived felsic
potassic rocks can also track redox conditions of mantle source. Our
results also suggest that, Eocene post-collisional mantle-derived
potassic magmas in western Yangtze Craton are oxidized in nature, which
are crucial for post-collisional porphyry Cu‒Au formation.