WATERVILLE – An investment banker and Colby College trustee has donated $6 million for construction of a new social sciences building for his alma mater.

The donation by Robert Diamond, chief executive of London-based Barclays Capital, is one of the largest lump sum gifts in school history.

“This gift is remarkable both in its magnitude and its capacity to change Colby for the better,” Colby President William Adams said Thursday. “The gift lays the groundwork for a broader campaign.”

The announcement comes less than a month after the college unveiled a $26.2 million plan to add four buildings to the Mayflower Hill campus. Three of the buildings will be built on the proposed Colby Green, across from the existing academic quadrangle.

The $12 million social sciences building will house the existing Oak Institute for the Study of International Human Rights, the yet-to-be-created Center for Public Affairs and Civic Engagement and several social science departments.

The Center for Public Affairs will draw together faculty from several existing programs such as government and environmental science to search for solutions to social problems, college spokesman Stephen Collins said.

Diamond was granted naming rights for the building, although no decision on a name has been made. Naming rights at the school are granted to donors who contribute 50 percent or more of the construction costs for a building.

Diamond graduated from Colby in 1973 and has served on the school’s board of trustees for nine years. During the college’s last capital campaign nearly a decade ago, Diamond donated more than $1 million to create a faculty position in the humanities division.