datasheet1_Petrogenesis of Mesoproterozoic Granites of the Swartoup Hills Region, Kakamas Domain, Namaqua Belt, South Africa.xlsx

The Swartoup and Polisiehoek plutons in the Swartoup Hills (South Africa) formed during an episode of significant magma emplacement in the Mesoproterozoic Namaqua Sector of the Namaqua Metamorphic Province. They intruded into mid-crustal metasedimentary rocks of the metapelitic Koenap and mafic to carbonate-bearing Bysteek Formations during and shortly after the ∼1,200–1,220 Ma regional metamorphic peak that reached ultrahigh temperatures. Subsequent to pluton emplacement, the crust underwent regional high-temperature deformation during slow near-isobaric cooling. A further episode of pluton emplacement associated with fluid circulation truncated the first-order regional tectonic structures at ∼1,100 Ma. Based on their petrography, the Swartoup pluton is subdivided into leuco-granitoids with biotite as the sole mafic phase, pyroxene granitoids, and garnet-bearing granitoids, which may contain significant biotite. These subgroups display distinctive geochemical variations from one another, and from the Koenap Formation migmatites and the Polisiehoek granites, which are exposed nearby. Incompatible trace element distributions suggest that the Swartoup and Polisiehoek granitoids represent modified A-type granite magma, consistent with derivation from partial melting of quartzo-feldspathic crust. The magmas incorporated significant amounts of juvenile mantle-derived magma (εNd1200 of ∼−5, and LREE-depleted), but do not require older, early to late Paleoproterozoic crust. Particularly close to contacts to the calcic Bysteek Formation, localized contamination of the Swartoup granites by supracrustal carbonates is evident. A relatively pervasive alkali metasomatic effect is manifested strongly in the initial 87Sr/86Sr and LILE profiles of the Polisiehoek granites in particular, as well as in some of the Swartoup pyroxene granitoids, which could be either a symptom of CO2 metasomatism related to the Bysteek Formation carbonates, or to post-magmatic fluid metasomatism, perhaps linked to regional shearing. The comparison of our results with literature data suggests that similar sources, A-type granitic, Meso- to Paleoproterozoic crustal, and enriched mantle, have contributed, in locally differing proportions, to granites in most parts of the Namaqua Sector. Most likely, these plutons were generated during crustal and mantle melting in a long-lived hot continental back-arc environment.

位于南非斯瓦特奥普山（Swartoup Hills）的斯瓦特奥普与波利斯谢霍克深成岩体（pluton），形成于纳马夸变质省（Namaqua Metamorphic Province）中元古代（Mesoproterozoic）纳马夸区块（Namaqua Sector）一期强烈的岩浆侵位事件中。它们侵入于达到超高温的~1200~1220 Ma区域变质峰期期间及之后不久的泥质变质科纳普组（metapelitic Koenap Formation）与基性至含碳酸盐比斯特耶克组（mafic to carbonate-bearing Bysteek Formations）的中地壳沉积变质岩中。岩体侵位后，地壳在缓慢近等压冷却（near-isobaric cooling）过程中发生了区域高温变形。约1100 Ma时，伴随流体循环的又一期岩浆侵位事件改造了一级区域构造结构。依据岩石学（petrography）特征，斯瓦特奥普岩体可划分为三类：以黑云母（biotite）作为唯一铁镁矿物的浅色花岗岩类（leuco-granitoids）、辉石花岗岩类（pyroxene granitoids）以及可含大量黑云母的含石榴子石花岗岩类（garnet-bearing granitoids）。这些亚类彼此之间，以及与附近出露的科纳普组混合岩（migmatite）和波利斯谢霍克花岗岩，均表现出显著的地球化学差异。不相容微量元素（incompatible trace element）分布特征显示，斯瓦特奥普与波利斯谢霍克花岗岩类属于改造型A型花岗岩（A-type granite）岩浆，其起源于长英质地壳（quartzo-feldspathic crust）的部分熔融。这些岩浆混入了大量新生幔源岩浆（juvenile mantle-derived magma，εNd(1200Ma)约为-5，且轻稀土亏损（LREE-depleted）），但并不需要古老的古元古代早-晚期地壳物质参与。尤其在与含钙质比斯特耶克组的接触带附近，斯瓦特奥普花岗岩被表壳碳酸盐岩（supracrustal carbonates）局部混染的现象十分显著。相对普遍的碱交代作用（alkali metasomatic effect）在波利斯谢霍克花岗岩的初始87Sr/86Sr比值与大离子亲石元素（LILE, Large Ion Lithophile Element）剖面中表现尤为强烈，部分斯瓦特奥普辉石花岗岩类也存在该现象；该作用可能与比斯特耶克组碳酸盐岩相关的CO₂交代作用有关，或是与区域剪切作用（regional shearing）相关的岩浆期后流体交代作用所致。将本次研究结果与文献数据（literature data）对比后可知，纳马夸区块大部分区域的花岗岩均由相似的源区贡献：A型花岗岩质、中元古代至古元古代地壳以及富集地幔，只是各源区的贡献比例存在区域差异。这类岩体最有可能形成于长期活跃的大陆弧后环境（long-lived hot continental back-arc environment）中的地壳与地幔熔融过程。