蛇綠岩為逆衝至陸殼上的海洋岩石圈殘片，保存了重建區域構造演化所需的關鍵地質紀錄。沙巴北部發育了一系列具代表性的蛇綠岩組合，主要包括白堊紀的古達蛇綠岩與馬魯都灣蛇綠岩（Kudat and Marudu Bay Ophiolites；KO–MBO），以及始新世的邦吉島蛇綠岩（Banggi Island Ophiolite；BIO）。這些岩體以往被歸入區域性的中生代至新生代「矽質頁岩–玄武岩組（Chert–Spilite Formations）」，亦稱為沙巴蛇綠岩，並被解釋為古南海（Proto–South China Sea，PSCS）海洋地殼的殘餘物。整體而言，它們記錄了巽他陸塊東南緣於中生代至新生代期間複雜的海洋岩漿活動與構造重組歷史。本論文整合了野外地質調查、岩石學、全岩地球化學、鋯石鈾鉛定年及鋯石鎦鉿同位素資料，以界定邦吉島、古達及馬魯都灣蛇綠岩的形成時代、岩漿成因與構造意義。
綜合上述結果，本研究提出：始新世的邦吉島蛇綠岩代表古南海隱沒起始期間新生成的弧前海洋地殼，而白堊紀的古達與馬魯都灣蛇綠岩則代表參與該隱沒系統的東亞海（East Asian Sea）上部板塊殘餘。沙巴北部同時保存白堊紀與古近紀蛇綠岩，反映海洋地殼生成具有時間差異，並與隱沒起始的逐步演化及其向巴拉望島與民都洛島的東北向遷移有關，最終導致沙巴–卡加延火山弧的形成。最後，本研究認為邦吉島蛇綠岩的形成時序與沙巴及鄰近區域廣泛分布的始新世不整合密切相關，反映了古南海隱沒起始所引發的重大區域構造重組事件。 關鍵詞：沙巴（婆羅洲）、鋯石鈾鉛定年、地球化學、隱沒帶上盤型蛇綠岩（SSZ 型）、隱沒起始、異質地函、古南海、卡加延海脊、始新世不整合;ABSTRACT
Ophiolites represent fragments of obducted oceanic lithosphere that preserve key geological records essential for reconstructing regional tectonic evolution. Northern Sabah hosts a distinctive assemblage of ophiolite complexes， including the Cretaceous Kudat and Marudu Bay Ophiolites (KO–MBO) and the Eocene Banggi Island Ophiolite (BIO). These complexes are traditionally regarded as part of the regional Mesozoic-Cenozoic “Chert-Spilite Formations” (also known as Sabah Ophiolite) and interpreted as remnants of the proto-South China Sea (PSCS) oceanic crust. Together， they record a complex history of Mesozoic–Cenozoic oceanic magmatism and tectonic reorganisation along the southeastern margin of Sundaland. This thesis integrates field observations， petrography， whole-rock geochemistry， zircon U–Pb geochronology， and zircon Lu–Hf isotopic data to constrain the age， petrogenesis， and tectonic significance of the BIO and KO–MBO. New high-precision zircon U–Pb ages demonstrate that the BIO formed during the Early Eocene (~55–50 Ma)， establishing the first robust crystallisation ages for this ophiolite. Zircon ƐHf(t) values from +16.6 to +10.2 indicate magma derivation from a relatively depleted or juvenile mantle source. The BIO crustal sequence including basalt， gabbro， and diorite exhibits two geochemically distinct types， a depleted type (i.e.， depleted in light rare earth elements) and an enriched type， respectively. The majority of BIO samples from both types display various degrees of enrichment in large-ion lithophile elements (LILE) coupled with systematic depletion in high-field-strength elements (HFSE) and elevated Th/Yb ratios， indicating a supra-subduction zone affinity. These geochemical and isotopic characteristics are consistent with formation in a forearc or proto-arc setting during the earliest stages of intra-oceanic subduction initiation， interpreted here to be associated with southward subduction of the PSCS. In contrast， zircon U–Pb ages from gabbroic to dioritic rocks of the KO–MBO indicate Early Cretaceous magmatism (~128–117 Ma). Their uniformly high and positive zircon ƐHf(t) values (average: +19 to +16; n = 9) indicate derivation from a relatively depleted mantle source. Mafic rocks from the KO–MBO exhibit geochemical signatures ranging from N-MORB to E-MORB and OIB affinities， lack systematic HFSE depletion， and show wide variations in incompatible element ratios (e.g.， Nb/Yb， Zr/Y， Nb/Y， Nb/La， and La/Sm). These characteristics are best explained by variable degrees of partial melting of a heterogeneous mantle source in a subduction-unrelated spreading ridge environment， possibly associated with near-axis seamount magmatism. More evolved dioritic and quartz dioritic rocks from the Marudu Bay Ophiolite display arc-like or adakitic geochemical signature; however， petrographic evidence and rare earth element systematics demonstrate that these features were produced by hornblende-dominated fractional crystallisation rather than slab melting or direct subduction input. Based on these results， this study argues that the Eocene BIO represents newly formed forearc oceanic crust generated during PSCS subduction initiation， whereas the Early Cretaceous KO–MBO represent fragments of the overriding East Asian Sea plate involved in this subduction system. The coexistence of Cretaceous and Paleogene ophiolites in northern Sabah reflects diachronous oceanic crust formation linked to progressive subduction initiation and subsequent northeastward propagation towards Palawan and then Mindoro， eventually leading to development of the Sabah–Cagayan volcanic arc. Finally， the timing of BIO formation is proposed to be closely related to the regional Eocene unconformity in Sabah and adjacent regions， reflecting a major tectonic reorganisation associated with PSCS subduction initiation.
Keywords: Sabah (Borneo)， Zircon U-Pb Geochronology， Geochemistry， SSZ-Type Ophiolite， Subduction Initiation， Heterogeneous Mantle， Proto-South China Sea， Cagayan Ridge
