The Xenocrysts Entrained From Diamondiferous Rocks:Implications For The Evolution Of The Deep Continental Crust Beneath The Southern Yangtze Block

The diamond-related mantle-derived magmatic rocks have a very deep origin,and the magmas rise rapidly.These rocks preserve significant information about the deep Earth and are considered as a unique window for obtaining knowledge of the internal Earth.Kimberlites and lamproites act like conveyor belts,transporting fragments of the lithosphere to the Earth's surface in time spans of hours to days.They often contain diverse crust-mantle xenoliths/xenocrysts that record the nature and evolution of the lithosphere.For example,the zircon xenocrysts can be used as probes of the deep continental crust,with implications for the formation and evolution events of the continent.In this Ph.D.thesis,it present the petrography,mineralogy,petrology,geochemistry,chronology,etc.studies for mantle-derived diamondiferous rocks?kimberlites and lamproites?and its crust-mantle xenocrysts?diamonds,garnets and zircons?in the east of Guizhou Province.Based on the previous research results,it reveals the formation and evolution of the deep crust in the southern Yangtze Block and discusses the multi-stage modification of the deep-seated lithosphere of the Yangtze continent by comparing their characteristics of petrology and geochemistry,crustal growth and reworking processes in the different spatial-temporal background.The Maping diamond-bearing rocks relatively resemble the kimberlites in general and have petrologically typical characteristics of the kimberlites,whereas the Baifen samples are more similar to the petrological and geochemical features of the lamproites and can be determined as lamproites.The element geochemical compositions of the Baifen lamproites characterized by high K₂O?3.94–5.07 wt.%?and MgO-rich?7.84–14.05 wt.%?contents,low Na₂O?0.21–0.28 wt.%?,Al₂O₃-poor?6.10–7.20 wt.%?and Fe₂O₃^T?7.79–8.87 wt.%?contents;rich in TiO₂?2.69–3.23 wt.%?,poor in CaO?6.87–13.86 wt.%?.They have lower SiO₂ contents?41.25–45.66 wt.%?and higher Mg^#?65.74–76.86?,which are indicative of ultramafic rocks.They are peralkaline[molar?Na₂O+K₂O?/Al₂O₃=0.67–0.96]and ultrapotassic?molar K₂O/Na₂O=9.75–15.52?rocks,yielding the typical Niggli k?0.91–0.94?and Niggli mg?0.79–0.87?parameters,which are in good agreement with the geochemical criteria for lamproite recognition.The Baifen lamproites have high Cr?452–599 ppm?,Ni?485–549 ppm?,Co?52.4–68.9ppm?and V?114–133 ppm?abundances,as well as Ba?1884–3589 ppm?,La?160–186ppm?,Sr?898–1152 ppm?and Zr?532–632 ppm?contents.The rocks display similar REE distribution patterns that are strongly enriched in the light REE[?La/Yb?_N=71.1–87.6]relative to the heavy REE.Their chondrite-normalized REE plots show right-inclined and linear patterns without Eu anomalies?Eu/Eu*=0.93–1.04?.In the PM-normalized trace elements spider diagram,the suites of Baifen lamproites have no obvious anomalies in Nb,Ta,Pb and Ti,which are very distinct from the continental crust with both notably negative Nb,Ta anomalies and an exclusively positive Pb anomaly.It suggests that crustal contamination appears to have little influence on the composition of the Baifen lamproites.The trace elements of the Baifen lamproites do not characterize those of the orogenic?Mediterranean and Tibet?lamproites with obvious positive Rb,Th,U and Pb anomalies,as well as negative Ba,Nb and Ti anomalies,pointing to the Baifen lamproites are very close to the features of the anorogenic type.The Maping kimberlites generally contain associated minerals of Cr-pyrope with high CaO contents,most of which belong to the G9?lherzolite?type rather than the global diamond-rich harzburgite protolith type?G10?,suggesting the state of lithosphere had been partly modified at that time and may not be conducive to the formation of high-quality diamonds.Integrated external morphology,internal structure,in situ U–Pb dating and Hf-isotope analyses of zircon xenocrysts from the Maping diamondiferous kimberlites and the related Baifen lamproites of the Zhenyuan area in the southeastern Guizhou Province offer a unique glimpse into the evolution of the deep continental crust beneath the southern part of the Yangtze Block,South China.Most zircons with pre-eruption ages are xenocrysts derived from the deeper crust of the southern Yangtze Block.Both the U–Pb ages of Archean zircons and the Archean Hf model ages(T_crust)of numerous Paleoproterozoic zircons demonstrate the presence of Archean crustal components?or remnants?as old as 3.7 Ga beneath the southern Yangtze Block.In addition,the zircon xenocrysts also record multiple post-Archean tectonothermal events,including the addition of juvenile mantle-derived materials at 2.2–2.1 Ga,2.0–1.8 Ga,1.1–0.6 Ga and284–264 Ma,and the reworking of old crustal components at 1.9–1.75 Ga,1.2–0.35 Ga and 275–252 Ma,suggesting that the unexposed Archean crustal relicts evolved through episodic crustal growth and reworking.Overall,our results imply that accretion/reworking processes beneath the southern Yangtze Block were different from those that affected the Yangtze continental nucleus?i.e.,the Kongling terrain?.However,the two youngest magmatic zircons give a weighted mean ²⁰⁶Pb/²³⁸U age of 253.0±2.8Ma,which is interpreted as the emplacement age of the host Baifen lamproites.We propose a possible linkage between the Baifen lamproite magmatism and the Emeishan flood-basaltvolcanism.