Foreword

Abstract

This is the second of three issues of American Journal of Science in which the papers have been submitted by friends and colleagues as a mark of respect to Alfred Kroner, who passed away just a few months before his 80 birthday in 2019. Alfred has left an indelible mark on the geoscience community, with his boundless energy and dedication, and especially his application of detailed fieldwork and microanalysis to solve the bigger issues. As the subtitle ‘From Cratons to Orogens’ indicates, Alfred was always cognizant of how his work related to global events, and this is reflected by the nine papers that constitute this volume. The first paper by Allen Nutman and co-workers presents an overview of the 3.8 Ga Outer Arc of the Isua Greenstone Belt in West Greenland. This forms an integral part of the oldest and best-preserved volcano-sedimentary sequence on Earth. Utilizing rare low-strain zones, accompanied by new and published geochronology, the authors endeavor to unravel the stratigraphy of the arc. The lowermost mafic unit contains pillow lavas, indicating a subaqueous depositional environment. The overlying formation commences with a unit of fuchsitic quartzite, overlain by various carbonate and silicate units; some with relict sedimentary structures. There is an upward transition to chemical sediments and then into units showing a greater influx of felsic volcanic components. The uppermost unit is felsic, with both lavas and pyroclastic rocks, interpreted as forming in an arc. The authors therefore support a plate tectonic scenario and not a stagnant lid environment for the early Eoarchean of Greenland. Xiao Wang and co-workers investigate Archean/Paleoproterozoic rocks in the Khondalite Belt, which marks the Paleoproterozoic collision zone between the Yinshan and Ordos blocks, and is located in the western block of the North China Craton. The authors describe TTG rocks of I-type affinity in the Daqingshan Complex, whose ages straddle the Archean/Proterozoic boundary and define oceanic subduction beneath the southern margin of the Yinshan Block. Docking of the Ordos Block led to closure of the ocean with the TTG rocks undergoing high-grade metamorphism between 1.95-1.85 Ga. In a companion paper, Xiao Wang and co-workers analyze and date high-grade metamorphic rocks of the Daqingshan Complex, interpreted as bimodal volcanics that formed in a back arc setting prior to the collision of the Yinshan and Ordos block, as detailed above. The volcanic rocks formed between 2.47-2.40 Ga and were metamorphosed at high grade between 1.95-1.85 Ga, similar to the TTG rocks in the complex. This latter event is one of the global collisions marking assembly of Colombia/Nuna. In another paper on the North China Craton where Alfred worked extensively, Yue-Lan Kang and co-workers investigate the late Mesozoic granitoids to the east and west of the Tan-Lu Fault, one of the most significant late Phanerozoic structures in China. In the Taihang Mountains to the west, granitoid magma is interpreted to have formed by a mixture of basaltic underplate and melt derived from the lower crust. To the east of the fault at Yunmengshan, the magmas were more variable, interpreted as being related to Paleo-Pacific subduction and the onset of lithospheric delamination beneath the eastern North China Craton. The authors further argue that the Tan-Lu Fault zone was a controlling factor in transporting both magma and fluids from the mantle. The final paper on Chinese geology takes us to the North Altyn Tagh Orogen in the SE of the Tarim Craton. Here, Qian Liu and co-workers use whole-rock and mineral geochemistry, together with zircon U/Pb geochronology, to investigate one of a series of early Paleozoic ophiolites. The Hongliugou ophiolitic mélange contains 490 Ma gabbro and 480 Ma diabase, derived from a depleted MORB-source mantle, variously metasomatized by subduction-related fluids. When combined with results from other ophiolites in the area, they argue that 520-510 Ma ophiolites represent initial subduction, the 490-480 Ma ophiolites