Episodic magmatism across the Permian-Triassic Crisis Boundary and its effects on underlying source rocks: records from conodont-based high-resolution geochemical proxies in marine carbonate successions, South China

Abstract

Abstract Magmatism of various magnitudes or intensities was widely recognized worldwide in accompanying with the end-Permian mass extinction (EPME) event across the Permian/Triassic boundary (PTB). Meanwhile, hydrocarbon source rocks were pervasively occurring in later-Permian marine carbonate successions. The EPME-related magmatism and later-Permian source rocks were associated spatially and temporally. However, the features of this magmatism and its effects on underlying source rocks were not elucidated. The current study investigated episodes and magnitudes of the magmatism across the PTB from typical South-China profile (i.e., the Pingdingshan section) using conodont-based geochemical proxies at a high-resolution scale (~50 kyr). Integrated trace elemental (Mn, Sr, Rb, and Th) and stable/radioactive isotopic (δ18O, δ13C, and 87Sr/86Sr) results revealed that conodonts provided an ideal proxy for chemostratigraphic signatures of ancient seawater, largely because it was more resistant to diagenetic alterations or thermal recrystallization. The conodont-based high-resolution 87Sr/86Sr values from studied interval (250.50 Ma to 252.00 Ma) showed three decreasing cycles upwardly against a long-term increasing background across the PTB, reflecting three episodes of magmatism. By contrary, the δ18O of same resolution and from same interval displayed no similar trend. This inconsistency was probably because that the δ18O composition of carbonates from studied section was limitedly altered due to long distance from magmatism center and/or buffering from thick water column. The micrite-based high-resolution δ13C exhibited an evolving pattern consistent with long-term background, revealing that the δ13C signatures of multiple stages of magmatism during this short-term interval were not inherited by micrites. The episodes of magmatism across the PTB can be correlated to underlying Chihsian source rocks from studied section according to the clustering of oxygen and strontium isotopic compositions of two sets of strata that were spatially and temporally related. The EPME-related magmatism across the PTB exerted great influences on formation of underlying marine source rocks by bring massive heat and pervasive oceanic anoxia.