Supplemental Material: From subduction initiation to hot subduction: Life of a Neoarchean subduction zone from the Dengfeng Greenstone Belt, North China Craton

We report a spatially and temporally linked arc magmatic association of 2.55−2.50 Ga mid-ocean ridge basalt (MORB)-affinity gabbros, arc-affinity basalts, high-Mg basalts/basaltic andesites (HMBA), Nb-enriched basalts (NEB), and a tonalite-trondhjemite-granodiorite (TTG)-like granodiorite from the Dengfeng greenstone belt (DFGB), North China Craton. These are identical to arc magmatic rock suites from hot subduction in the present plate mosaic, and resemble suites formed during subduction initiation. Laser ablation−inductively coupled plasma−mass spectrometry zircon U-Pb dating indicates that gabbros and granodiorites intruded at ca. 2.55 Ga and the HMBAs formed at ca. 2.50 Ga, indicating a long duration of subduction zone magmatic activity of at least 50 m.y. in the DFGB. The gabbros are characterized by flat light-rare earth element (LREE) patterns without negative Nb and Zr anomalies, showing a MORB-like geochemical affinity. Combined with previously reported normal-MORB-type basalts, we suggest that they may represent magmatic products during intraoceanic subduction initiation where their mantle source was slightly metasomatized by subduction-derived fluids during the initiation stage. The arc-affinity basalts are characterized by slightly enriched LREE patterns with negative Nb and Zr anomalies. The HMBAs have higher contents of MgO, Ni, and Cr and display more fractionated rare earth element (REE) patterns and large negative Nb and Zr anomalies than the arc-affinity basalts. The NEBs are characterized by high absolute contents of Nb (9.9−14 ppm) and high ratios of Nb/Laprimitive mantle (pm) (0.40−0.46) and Nb/Thpm (0.44−0.54), and moderately fractionated REE and minor negative Nb anomalies with no Zr anomalies. One granodiorite sample is characterized by a highly fractionated REE pattern and negative Nb and Zr anomalies. Geochemical modeling suggests that the association of the normal arc-affinity basalt (NAB)-HMBA-NEB can be interpreted to be generated by hybridization of mantle wedge peridotites by slab-derived TTG-like melts. Positive values of whole-rock εNd (t) and zircon εHf (t) of the 2.55−2.50 Ga magmatic associations are consistent with a long-term depleted mantle source for the mafic assemblage. The 2.55−2.50 Ga magmatic association from MORB-affinity gabbros and basalts to arc-affinity NAB-HMBA-NEB may record a continuous Neoarchean geodynamic process from intraoceanic subduction initiation to mature arc magmatism. The NAB-HMBA-NEB association with regional sanukitoids and high-Al TTGs indicates that hot subduction may have played an important role in the production of arc-related magmatism during the Archean.

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Life cycle of an Archean subduction zone from initiation to arc–polarity reversal: Insights from the Zunhua ophiolitic mélange, North China Craton

10.5194/egusphere-egu21-3833 ◽

2021 ◽

Author(s):

Wenbin Ning ◽

Timothy Kusky ◽

Junpeng Wang ◽

Lu Wang ◽

Hao Deng ◽

...

Keyword(s):

Life Cycle ◽

Subduction Zone ◽

North China Craton ◽

Plate Tectonics ◽

North China ◽

Polarity Reversal ◽

Iron Formation ◽

Arc Magmatism ◽

Subduction Initiation ◽

Depleted Mantle

<p>Subduction initiation and arc&#8211;polarity reversal have rarely been recognized in the Archean rock record. We document Neoarchean subduction initiation, fore-arc magmatism, and an arc&#8211;polarity reversal event from the Zunhua structural belt along the eastern margin of the Central Orogenic Belt (COB) of the North China Craton (NCC). The Zunhua ophiolitic m&#233;lange within the Zunhua structural belt is a mappable unit characterized by blocks of metamorphosed harzburgite/lherzolite, podiform chromite &#8211;bearing dunite, pyroxenite, amphibolite, metabasites (basalt and diabase) with rare intermediate volcanics, chert, and tectonic lenses of banded iron formation in a strongly sheared metapelitic matrix. New geochronological and geochemical analyses of magmatic blocks within the ophiolitic m&#233;lange show that the crustal magmatic rocks were produced in a fore-arc region at 2.55&#8211;2.52 Ga from depletion of the harzburgitic&#8211;lherzolitic mantle tectonites. Chemical, petrological, and temporal links between the depleted mantle blocks, and the suite of magmatic blocks derived from partial melting and metasomatism of these depleted mantle blocks, unequivocally shows that they represent part of the same original Neoarchean fore-arc ophiolite suite. After formation and accretion in the oceanic realm, the m&#233;lange was emplaced on the continental margin of the Eastern Block between 2.52&#8211;2.50 Ga, and underwent two stages of metamorphism at ca. 2.48&#8211;2.46 Ga and 1.81 Ga. Metamorphosed intermediate&#8211;mafic volcanic blocks exhibit systematic successive geochemical variations, from MORB-like to volcanic arc-like, and the N-MORB-like meta-basalts show remarkable similarity with the subduction initiation-related Izu&#8211;Bonin&#8211;Mariana (IBM) fore-arc basalts. We suggest that the Zunhua fore-arc complex records continuous geodynamic processes from subduction initiation to arc magmatism. The Zunhua ophiolitic m&#233;lange is part of a ca. 2.5 Ga suture between an oceanic arc of the COB and Eastern Block of the NCC. After the arc&#8211;continent collision, an arc&#8211;polarity reversal event has been proposed to initiate a new eastward&#8211;dipping subduction zone on the western side of the COB. This arc&#8211;polarity reversal can be traced for more than 1,600 km along the length of the orogen, similar in scale, geometry, and duration between collision and polarity flip to the present-day arc&#8211;polarity reversal of the Sunda&#8211;Banda arc during its ongoing collision with the Australia continent. This indicates that a life cycle of an Archean subduction zone, including birth (subduction initiation), maturity (arc magmatism), death (arc-continent collision) and re-birth (arc&#8211;polarity reversal), is recorded in the Zunhua ophiolitic m&#233;lange, and the geodynamics of plate tectonics at the end of the Archean was similar to that of today.</p><p>&#160;</p>

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