关键词: 片麻岩, TTG, 元素地球化学, 热力学模拟, 分离结晶, 固阳

Abstract:  In Guyang, Inner Mongolia, a significant occurrence of Late Neoarchean (quartz) diorite-tonalite-trondhjemite-granodiorite (DTTG) rock assemblages has been identified. LA-ICP-MS zircon U-Pb dating has yielded crystallization ages of (2 504±11) and (2 514±7) Ma for the quartz dioritic and tonalitic gneisses, respectively. Additionally, the dioritic gneisses exhibit inherited zircon ages of ca. 2.6 Ga. The quartz dioritic (D) gneisses in Guyang display a relatively low w(SiO2) (62.08%-62.62%), Sr/Y ratios (23.3-39.8), and elevated concentrations of w(MgO) (2.08%-3.06%) and w(TFe2O3) (5.43%-6.01%). Notably, these rocks demonstrate either negligible or weak Eu negative anomalies (Eu/Eu* = 0.91-0.98) and show subtle differentiation between light and heavy rare earth elements ((La/Yb)N = 7.59-21.60). These features are indicative of island arc calc-alkaline geochemical signatures. Combining these characteristics with zircon Hf isotope compositions suggests that the late Neoarchean diorites likely formed through crustal partial melting under relatively low-pressure and shallow conditions. In contrast, the late Neoarchean tonalitic-trondhjemitic (TT) gneisses in the Guyang area exhibit higher w(SiO2) (67.95%-77.01%), elevated Sr/Y ratios (120.00-941.00), and lower w(MgO) (0.14%-1.29%) and w(TFe2O3) (0.63%-2.66%). They also feature significant Eu positive anomalies (Eu/Eu* = 1.48-8.73) and prominent differentiation between light and heavy rare earth elements ((La/Yb)N = 29.70-140.00). While the consensus attributes the origins of TTG rocks to partial melting of hydrated altered basalts, the geochemical signatures of the   Late Neoarchaean (quartz) diorite-tonalite-trondhjemite (DTT) rocks in the Guyang area allude to fractional crystallization of amphibole. A mixing simulation involving Guyang cumulative hornblendite and trondhjemite accurately reproduces the composition of quartz dioritic gneisses in the Guyang area. Thorough thermodynamic and trace element quantitative modelling reveals that the Late Neoarchean tonalic-trondhjemitic rocks likely formed through multistage fractional crystallization from dioritic magma in a water-rich and relatively low-pressure condition. Amphibole is the primary mineral phase involved in fractional crystallization, with minor amounts of plagioclase and sphene. The high whole-rock Sr/Y and Eu/Eu* values do not indicate sufficient conditions for a relatively high-pressure source.

Key words:  , gneiss, TTG, elemental geochemistry, thermodynamic modelling, fractional crystallization, Guyang