Post-orogenic alkaline granites from China and comparisons with anorogenic alkaline granites elsewhere

Abstract

Alkaline granites can be broadly divided into anorogenic (AA-type) and post-orogenic (PA-type). The former heralds the beginning of rifting and the latter is an indicator of the end of orogeny. Although it is difficult to distinguish anorogenic and post-orogenic alkaline granites, they can be identified on the basis of differences in geology, petrology and geochemistry: (1) PA-type granites are formed shortly after plate subduction and plate collision; AA-type granites are independent of subduction and collision. (2) AA-type granitic magmatism is of long time duration; PA-type granitic magmatism is generally short-lived. (3) AA-type granites tend to be associated with lithospheric rifting; PA-type granites are commonly associated with ophiolitic plate suture zones. (4) AA-type granites are often closely associated with mafic rocks and silica-undersaturated syenites; PA-type granites are the final products of a long tectonic magmatic cycle dominated by a normal calc-alkaline series. (5) AA-type granites have an extended range in R₁ (500–3000; the range for PA-type granites is very narrow RC 2300–2600). The $\text{Ga}\text{Al}$ ratio of the former is generally 4–9, while that of the latter is 2–4. The former is more enriched in Zr, Nb, Ce and Y and has smaller $\text{Y}\text{Nb}$ and $\text{Ce}\text{Nb}$ ratios than the latter.