Cretaceous intraplate volcanism of Govorov Guyot and formation models of the Magellan seamounts, Pacific Ocean

The OIB-type volcanic rocks of Govorov Guyot in Magellan Seamounts (Pacific Ocean) include basalts, trachybasalts, basaltic trachyandesites, and trachyandesites dated by the 40Ar-39Ar plateau from 121 ± 2.8 to 98.5 ± 1.4 Ma. Some rocks bear amphibole-spinel wehrlite xenoliths and clinopyroxene-amphibole (Cpx-Amp) mantle vein fragments in xenoliths and xenocrysts, which are remnants of metasomatized oceanic lithosphere. The pressure and temperature crystallization for pargasitic amphibole were the highest (1170–1130°C and 2.5–1.6 GPa, 73–53 km depth) in wehrlite peridotite and the lowest (1.6–1.1 GPa, shallower than 52 km, and 1070–980°C) in Cpx-Amp metasomatic mantle veins. Ti-bearing Amp phenocrysts and microlites crystallized from the parent melts of basaltic rocks at 1060–910°C and 1.2–0.4 GPa (40–15 km). The ascending basaltic melts lost H2O from 8.6 ± 1.2 to 2.6 ± 0.4 wt% upon decompression. Amphibole mantle xenocrysts became replaced by Ti-magnetite- and rhönite-bearing mineral assemblages in intermediate magma chambers within oceanic crust (3–7 km). Pargasitic Amp in wehrlite and metasomatic mantle veins may have crystallized at different depths from hydrous (6–4.5 ± 0.7 wt% H2O) mafic melts that percolated through peridotite. The sources of heat and metasomatic agents (silicate melts and/or fluids enriched in volatiles) most likely had no relation to the Pacific Plate subduction but may be associated with the Southern Pacific Superplume in the South Pacific Thermal and Isotopic Anomaly region, or may be located at the Lithosphere-Asthenosphere Boundary (LAB) level in the plume absence. Repeated events of Cretaceous intraplate volcanism in the Magellan guyots can be explained in terms of plume (hotspot) activity or decompression partial melting of enriched or metasomatized asthenosphere and/or metasomatized oceanic lithosphere caused by Pacific Plate deformation and fracturing from the LAB level.太平洋麦哲伦海山（Magellan Seamounts）戈沃罗夫平顶海山（Govorov Guyot）的洋岛玄武岩（Ocean Island Basalt, OIB）型火山岩包含玄武岩、粗面玄武岩、玄武质粗面安山岩及粗面安山岩，采用40Ar-39Ar坪年龄法测年的结果介于121 ± 2.8 Ma至98.5 ± 1.4 Ma之间。部分岩石含有角闪石-尖晶石方辉橄榄岩捕虏体，以及捕虏体和捕虏晶中的单斜辉石-角闪石（clinopyroxene-amphibole, Cpx-Amp）幔源脉体碎片，这些均为经历交代改造的大洋岩石圈残余体。韭闪石（pargasitic amphibole）的结晶温压条件在方辉橄榄岩型捕虏体中最高（1170~1130℃、2.5~1.6 GPa，对应深度73~53 km），在单斜辉石-角闪石交代幔源脉体中最低（1.6~1.1 GPa、深度浅于52 km，温度1070~980℃）。含钛角闪石斑晶及微晶从玄武岩母浆中结晶的温压条件为1060~910℃、1.2~0.4 GPa（对应深度40~15 km）。上升的玄武质熔体在减压过程中H₂O含量从8.6 ± 1.2 wt%降至2.6 ± 0.4 wt%。在大洋地壳内部的中间岩浆房（深度3~7 km）中，幔源角闪石捕虏晶被含钛磁铁矿和黑铝镁钛矿（rhönite）的矿物组合所交代。方辉橄榄岩中的韭闪石及交代幔源脉体中的韭闪石，可能是由穿过橄榄岩的含水（H₂O含量6~4.5 ± 0.7 wt%）镁铁质熔体在不同深度结晶形成。热与交代介质（硅酸盐熔体和/或富挥发分流体）的源区大概率与太平洋板块俯冲作用无关，但可能与南太平洋热与同位素异常区的南太平洋超级地幔柱（Southern Pacific Superplume）有关，或是在无地幔柱情况下位于岩石圈-软流圈边界（Lithosphere-Asthenosphere Boundary, LAB）处。麦哲伦海山群白垩纪板内火山作用的多期事件，可通过地幔柱（热点）活动，或是由太平洋板块在岩石圈-软流圈边界处的变形和断裂引发的富集型/交代改造型软流圈减压部分熔融，以及交代改造后的大洋岩石圈熔融加以解释。