Impact‐induced melting of Archean granulites in the Vredefort Dome, South Africa. I: anatexis of metapelitic granulites
Impact‐induced melting of Archean granulites in the Vredefort Dome, South Africa. I: anatexis of metapelitic granulites
复制标题
南非 Vredefort Dome 中太古代麻粒岩的撞击引起熔化 I：变泥质麻粒岩的深熔作用。
DOI：
10.1046/j.0263-4929.2001.00358.x
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发表时间：
2002
期刊：
影响因子：
--
通讯作者：
R. Gibson
中科院分区：
文献类型：
--
作者：
R. Gibson
在2.02 Ga时，Vredefort穹丘中的中地壳太古代泥质麻粒岩经历了与陨石撞击形成穹丘相关的静态低P麻粒岩相叠加。 加热和折返几乎是瞬间的，主要热源是撞击冲击波非绝热衰变释放的能量。该结构中心几公里半径范围内的最高温度超过900 °C，局部甚至超过1350 °C。 这导致了前体太古代麻粒岩组合（Grt + Bt + Qtz + Pl + Ksp ± Crd ± Opx ± Sil）的全面熔融，随后是铝质碱性长石+Crd + Spl ± Crn ± Sil共生体的围晶结晶和小的、演化的黑云母浅色花岗岩体的分凝。 在距离C。距离中心6公里处的撞击前岩石特征基本上保留了下来，尽管石榴石被Crd + Opx ± Spl ± Pl的合晶冠和黑云母被斜方辉石部分取代，表明峰值温度接近775 ± 50 °C。 钾长石的薄间隙护城河与斜方辉石冕密切相关;它们被解释为黑云母分解产生的低比例部分熔体的残留物。纹理和矿物成分的数据支持这些岩石中的平衡体积减少，这反映了快速等压冷却后的冲击加热和折返。高温和强烈的横向热梯度与模拟的直径为200-300公里的撞击坑的撞击诱导等温线模式一致。
Mid‐crustal Archean pelitic granulites in the Vredefort Dome experienced a static, low‐P granulite facies overprint associated with the formation of the dome by meteorite impact at 2.02 Ga. Heating and exhumation were virtually instantaneous, with the main source of heat being provided by energy released from nonadiabatic decay of the impact shock wave. Maximum temperatures within a radius of a few kilometres of the centre of the structure exceeded 900 °C and locally even exceeded 1350 °C. This led to comprehensive melting of the precursor Archean granulite assemblages (Grt + Bt + Qtz + Pl + Ksp ± Crd ± Opx ± Sil) followed by peritectic crystallization of aluminous alkali feldspar+Crd + Spl ± Crn ± Sil parageneses and the segregation of small, evolved, biotite leucogranite bodies. However, at a distance of c. 6 km from the centre pre‐impact rock features are largely preserved, although partial replacement of garnet by symplectitic coronas of Crd + Opx ± Spl ± Pl and biotite by orthopyroxene indicate that peak temperatures approached 775 ± 50 °C. Thin interstitial moats of K‐feldspar are closely associated with the orthopyroxene coronas; they are interpreted as the remnants of low‐proportion partial melts generated by biotite breakdown. Both the textures and mineral compositional data support reduced equilibration volumes in these rocks, which reflect rapid isobaric cooling following shock heating and exhumation. The high temperatures and strong lateral thermal gradient are consistent with the modelled impact‐induced isotherm pattern for a 200–300 km diameter impact crater.