Geochemistry of marin ash layers and epiclastic rocks from the Kerguelen Plateau

Epiclastic volcanogenic rocks recovered from the Kerguelen Plateau during Ocean Drilling Program Legs 119 and 120 comprise (pre-)Cenomanian(?) claystones (52 m thick, Site 750); a Turonian(?) basaltic pebble conglomerate (1.2 m thick, Site 748; Danian mass flows (45 m thick, Site 747); and volcanogenic debris flows of Quaternary age at Site 736 (clastic apron of Kerguelen Island). Pyroclastic rocks comprise numerous Oligocene to Quaternary marine ash layers. The epiclastic sediments with transitional mid-ocean-ridge basalt (T-MORB) origin indicate weathering (Site 750) and erosion (Site 747) of Early Cretaceous T-MORB from a then-emergent Kerguelen Plateau, connected to Late Cretaceous tectonic events. The basal pebble conglomerate of Site 748 has an oceanic-island basalt (OIB) composition and denotes erosion and reworking of seamount to oceanic-island-type volcanic sources. The vitric- to crystal-rich marine ash layers are a few centimeters thick, have rather uniform grain sizes around 60 ± 40 µm, and are a result of Plinian eruptions. Crystal-poor silicic vitric ashes may also represent co-ignimbrite ashes. The ash layers have bimodal, basaltic, and silicic compositions with a few intermediate shards. The basaltic ashes are evolved high-titanium T-MORB; a few grains in a silicic pumice lapilli layer have a low-titanium basaltic composition. The silicic ashes comprise trachytic and rhyolitic glass shards belonging to a high-K series, except for a few low-K glasses admixed to a basaltic ash layer. Feldspar and clinopyroxene compositions fit the glass chemistry: high-Ti tholeiite-basaltic glasses have Plagioclase of An40-80 and pigeonite to augite clinopyroxene compositions. Silicic ashes have K-rich anorthoclase and minor Plagioclase around An20 and ferriaugitic to hedenbergitic clinopyroxene compositions. The line of magmatic evolution for the glass shards is not compatible with simple two-end member (high-Ti T-MORB and high-K rhyolite) mixing, but favors successive Ca-Mg-Fe pyroxene, Ti magnetite, and apatite fractionation, and K-rich alkali feldspar fractionation in trachytic magmas to yield rhyolitic compositions. Plagioclase fractionation occurs throughout. This qualitative model is in basic accordance with the observed mineral assemblage. However, as the time span for explosive volcanism spans >30 m.y., this basic model cannot comply with fractional crystallization in a single magma reservoir. The ash layers resulted from highly explosive eruptions on Kerguelen and, with less probability, Heard islands since the Oligocene. The explosive history starts with widespread Oligocene basaltic ash layers that indicate sea-level or subaerial volcanism on the Northern Kerguelen Plateau. After a hiatus of 24 m.y.(?), explosive magmatic activity was vigorously renewed in the late Miocene with more silicic eruptions. A peak in explosive activity is inferred for the Pliocene-Pleistocene. The composition and evolution of Kerguelen Plateau ash layers resemble those from other hotspot-induced, oceanic-island realms such as Iceland and Jan Mayen in the North Atlantic, and the Canary Islands archipelago in the Central Atlantic.本数据集涵盖大洋钻探计划（Ocean Drilling Program）第119、120航次从凯尔盖朗高原（Kerguelen Plateau）采获的两类岩石：其一为火山成因碎屑岩（epiclastic volcanogenic rocks），具体包括750站位的（前）森诺曼期（？）黏土岩（厚度52米）、748站位的土仑期（？）玄武岩砾岩（厚度1.2米）、747站位的达宁期块体流沉积（厚度45米），以及736站位的第四纪火山成因碎屑流（对应凯尔盖朗岛碎屑裙）；其二为火山碎屑岩（pyroclastic rocks），包含大量渐新世至第四纪的海相火山灰层。 具有过渡型洋中脊玄武岩（transitional mid-ocean-ridge basalt, T-MORB）成因的碎屑沉积，指示当时出露的凯尔盖朗高原早白垩世T-MORB经历了风化作用（750站位）与侵蚀作用（747站位），该过程与晚白垩世构造事件存在关联。748站位底部的玄武岩砾岩具有洋岛玄武岩（oceanic-island basalt, OIB）地球化学特征，代表海山至洋岛型火山源的侵蚀与再沉积作用。 富玻璃质至富晶屑的海相火山灰层厚度仅数厘米，粒度相对均一，集中于60±40μm区间，均形成于普林尼式喷发（Plinian eruptions）。贫晶屑硅质玻璃质火山灰也可归因为同火成碎屑灰。该火山灰层呈现双峰式组分特征，包含玄武质与硅质两个端元，仅夹杂少量中间型屑粒。其中玄武质火山灰为演化型高钛T-MORB；某一硅质浮石火山砾层中的少量碎屑具有低钛玄武质成分。硅质火山灰由粗面质与流纹质玻璃屑组成，归属高钾系列，仅在部分玄武质火山灰层中混杂有少量低钾玻璃质。 斜长石（Plagioclase）与单斜辉石（clinopyroxene）的地球化学特征与玻璃组分高度匹配：高钛拉斑玄武质玻璃对应An值40~80的斜长石，以及从古铜辉石至普通辉石的单斜辉石组分。硅质火山灰则含富钾歪长石，以及少量An值约20的斜长石，还有从富铁普通辉石至钙铁辉石的单斜辉石组分。玻璃屑的岩浆演化轨迹无法通过简单的两端元（高钛T-MORB与高钾流纹岩）混合来解释，而更支持以下过程：粗面质岩浆中依次发生钙镁铁辉石、钛磁铁矿与磷灰石的分异作用，以及富钾碱性长石分异，最终形成流纹质组分；斜长石分异贯穿整个演化过程。该定性模型与观测到的矿物组合基本吻合，但由于爆发式火山作用的持续时长超过30百万年，该基础模型无法用单一岩浆房内的分离结晶过程来解释。 上述火山灰层均形成于渐新世以来凯尔盖朗岛及概率较低的赫德岛（Heard Islands）上的强爆炸性喷发。其喷发历史以广泛分布的渐新世玄武质火山灰层为开端，指示凯尔盖朗高原北部存在海平面下或陆上火山活动。在经历约24百万年（？）的沉积间断后，中新世晚期爆发式岩浆活动再次活跃，伴随更为强烈的硅质喷发；上新世-更新世被认为是爆发活动的峰值期。 凯尔盖朗高原火山灰层的地球化学组成与演化特征，与其他热点驱动的洋岛区域高度相似，例如北大西洋的冰岛（Iceland）与扬马延（Jan Mayen），以及中大西洋的加那利群岛（Canary Islands）群岛。