Rocks of the LIC range in composition from troctolite and gabbro through diorite to trondhjemite and granite. All of the ultramafic rocks and most of the gabbros have petrographic and geochemical features consistent with a cumulate origin and mineral chemistry shows similarities with arc cumulate sequences from elsewhere. Few of the plutonic rocks making up the LIC have direct analogues among modern intra-oceanic volcanic rocks. The latter are the end products and the former the leftovers from magmatic processes that included fractional crystallisation, crustal assimilation and magma mixing and mingling. Longwood intrusions do not represent magma chambers. They formed as crystal cumulates and mushes left over from the processes that generated magmas erupted at the contemporary volcanic arc.
A correlation between decreasing age of emplacement and Sr and Nd isotopic compositions and inheritance in zircons dated by ion probe are indications of crustal recycling. The generation of felsic rocks in the Longwood intra-oceanic arc involved crustal anatexis and, over the 100 million year history of the arc, the crust evolved towards a composition similar to bulk continental crust and average andesite.
Dioritic rocks of the LIC contain abundant mafic enclaves, which are argued to represent fragments of mafic magma, derived by fractional crystallisation from basalt, which was intruded into a hot but solid or near solid diorite. Heating and remobilisation of the dioritic host disrupted and disaggregated the intruding mafic magma to form enclaves and zones of intrusion breccia that show every variation from liquid–liquid to liquid–solid mingling and mixing. They were then further modified chemically and mineralogically by diffusion of H2O, Na, P, Ba, REE, and, to a lesser extent, Rb.
Mafic dykes occur throughout the Complex and a number of these are composite with compositions ranging from dolerite through andesite to dacite. The components of composite dykes do not define unequivocal linear mixing trends and hybridisation processes that took place within them have only localised significance; mingling and hybridisation in the composite dykes do not appear to have controlled geochemical variation among the major intrusive units of the Complex.