Tracking Crystal‐Melt Segregation and Accumulation in the Intermediate Magma Reservoir

• Published in: Geophysical research letters Vol. 50; no. 10
• Main Authors: Ma, Jian‐Feng, Wang, Xiao‐Lei, Yang, Alexandra Yang, Zhao, Tai‐Ping
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 28.05.2023; Wiley
• Subjects: Accumulation; Amphiboles; Analogs; Chronostratigraphy; Continental crust; Cratons; crystal accumulation; Crystallization; Crystals; crystal‐melt segregation; Evolution; Geochemistry; Geochronology; Hygrometers; Hygrometry; intermediate intrusions; Lava; Magma; magmatic water; Melts (crystal growth); Minerals; Moisture content; Plutons; Reservoirs; Rock; Rocks; Segregation; Trace elements; Tracking; Viscosity; Water content; Water storage; Zircon; China
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2022GL102540

The genesis of intermediate intrusions is highly controversial, and one of the hot topics is whether they represent frozen melts or cumulates in the evolution of magmatic systems. Distinguishing accumulation from crystallization melt differentiated along the liquid line of descent is the key issue. The Paleoproterozoic intermediate intrusions in southern North China Craton provide an excellent case to decipher this issue. Multiple lines of evidence, including mineral textures, geochemistry as well as alphaMELTS modeling, indicate disequilibrium between whole‐rock and minerals, with melt extraction occurring at temperatures of 760°–820°C and with 10–40 wt.% of trapped melts. Effective water storage, revealed by amphibole and clinopyroxene hygrometers, plays a crucial role in promoting crystal‐melt segregation in pluton‐sized reservoirs in the upper crust. This study demonstrates that the accumulation in intermediate magmas can be identified even without evident complementary initial and extracted melts and provides deep insights into the genesis of intermediate continental crust. Plain Language Summary The genesis of intermediate rocks has long been controversial since they are analogs of the average composition of continental upper crust. The key topic is whether intermediate rocks represent frozen melts of intermediate magmas or crystal residues after melt extraction in the evolution of felsic magmatic systems. Here we carried out a comprehensive study on the Paleoproterozoic intermediate intrusions in the southern North China Craton, which is considered to have recorded the process of crystal accumulation and melt segregation in the shallow crust. Upon mineral texture, geochronologic, whole‐rock and mineral geochemical, and thermodynamic investigation, we propose that crystal accumulation and crystal‐melt segregation processes can be identified in intermediate magmas with the melt extraction temperatures and proportions of trapped melts recorded by zircon trace elements. Amphibole and clinopyroxene hygrometers revealed high water content of the melts, which likely decreased the viscosity of melts and facilitated effective cumulate‐melt segregation. This study provides a powerful reference to see through the accumulation in intermediate magmas, record the processes of crystal‐melt segregation, and definitively resolve the debate concerning the genesis of intermediate intrusions. Key Points Crystal accumulation in intermediate magmas can be identified by mineral texture and geochemistry The effective water storage system in the reservoirs is key to promoting crystal‐melt segregation Crystal‐melt segregation and accumulation are fundamental processes to form evolved continental upper crust